Merging to release-5.9.1: [TT-15507] Revert changes to the "/hello" health check endpoint (#7295)

[TT-15507] Revert changes to the "/hello" health check endpoint (#7295)

### **User description**

- The /hello (liveness) endpoint is reverted to always respond with HTTP
200, regardless of internal health check status.
- Health status calculation in the liveness handler is refactored and
simplified—now based solely on the number of failed checks (failCount),
removing the previous "critical failure" logic.
- Error handling is added for JSON encoding errors in the readiness
handler, so encoding issues are now logged.

<details open>
<summary><a href="https://tyktech.atlassian.net/browse/TT-15507"
title="TT-15507" target="_blank">TT-15507</a></summary>
  <br />
  <table>
    <tr>
      <th>Summary</th>
      <td>Revert changes to the "/hello" health check endpoint</td>
    </tr>
    <tr>
      <th>Type</th>
      <td>
<img alt="Bug"
src="https://tyktech.atlassian.net/rest/api/2/universal_avatar/view/type/issuetype/avatar/10303?size=medium"
/>
        Bug
      </td>
    </tr>
    <tr>
      <th>Status</th>
      <td>In Dev</td>
    </tr>
    <tr>
      <th>Points</th>
      <td>N/A</td>
    </tr>
    <tr>
      <th>Labels</th>
      <td>-</td>
    </tr>
  </table>
</details>
<!--
  do not remove this marker as it will break jira-lint's functionality.
  added_by_jira_lint
-->

---

<!-- Provide a general summary of your changes in the Title above -->

## Description

<!-- Describe your changes in detail -->

## Related Issue

<!-- This project only accepts pull requests related to open issues. -->
<!-- If suggesting a new feature or change, please discuss it in an
issue first. -->
<!-- If fixing a bug, there should be an issue describing it with steps
to reproduce. -->
<!-- OSS: Please link to the issue here. Tyk: please create/link the
JIRA ticket. -->

## Motivation and Context

<!-- Why is this change required? What problem does it solve? -->

## How This Has Been Tested

<!-- Please describe in detail how you tested your changes -->
<!-- Include details of your testing environment, and the tests -->
<!-- you ran to see how your change affects other areas of the code,
etc. -->
<!-- This information is helpful for reviewers and QA. -->

## Screenshots (if appropriate)

## Types of changes

<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Refactoring or add test (improvements in base code or adds test
coverage to functionality)

## Checklist

<!-- Go over all the following points, and put an `x` in all the boxes
that apply -->
<!-- If there are no documentation updates required, mark the item as
checked. -->
<!-- Raise up any additional concerns not covered by the checklist. -->

- [ ] I ensured that the documentation is up to date
- [ ] I explained why this PR updates go.mod in detail with reasoning
why it's required
- [ ] I would like a code coverage CI quality gate exception and have
explained why


___

### **PR Type**
Bug fix, Tests


___

### **Description**
Restore /hello to always return 200
Compute Pass/Warn/Fail from check counts
Add extensive liveness vs readiness tests
Improve JSON encode error handling logs


___

### Diagram Walkthrough


```mermaid
flowchart LR
  liveH["liveCheckHandler (/hello)"] -- "always 200" --> http200["HTTP 200"]
  liveH -- "derive Pass/Warn/Fail" --> status["Response.Status"]
  readyH["readinessHandler (/ready)"] -- "encode + log errors" --> logR["Warning on encode error"]
  tests["health_check_test.go"] -- "cover scenarios" --> liveH
  tests -- "compare vs readiness" --> readyH
```



<details> <summary><h3> File Walkthrough</h3></summary>

<table><thead><tr><th></th><th align="left">Relevant
files</th></tr></thead><tbody><tr><td><strong>Bug
fix</strong></td><td><table>
<tr>
  <td>
    <details>
<summary><strong>health_check.go</strong><dd><code>Rework liveness logic
and enforce 200 response</code>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; </dd></summary>
<hr>

gateway/health_check.go

<ul><li>Compute failCount inline and set status.<br> <li> Always write
HTTP 200 for /hello.<br> <li> Add error handling for JSON encode in
readiness.<br> <li> Remove determineHealthStatus usage in liveness.</ul>


</details>


  </td>
<td><a
href="https://github.com/TykTechnologies/tyk/pull/7295/files#diff-978a2d1427d9209765e541618af10683944c6396df1a6fb8b5221e4f16658a6a">+24/-4</a>&nbsp;
&nbsp; </td>

</tr>
</table></td></tr><tr><td><strong>Tests</strong></td><td><table>
<tr>
  <td>
    <details>
<summary><strong>health_check_test.go</strong><dd><code>Add
comprehensive liveness and readiness tests</code>&nbsp; &nbsp; &nbsp;
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; </dd></summary>
<hr>

gateway/health_check_test.go

<ul><li>Add tests for /hello status and body.<br> <li> Add liveness vs
readiness behavior tests.<br> <li> Add edge case coverage and headers
checks.<br> <li> Validate JSON payload fields and errors.</ul>


</details>


  </td>
<td><a
href="https://github.com/TykTechnologies/tyk/pull/7295/files#diff-08e29946afc7757a9c7baaef04b1a81964640437a684ff6306d1a0c933ac3f6a">+552/-0</a>&nbsp;
</td>

</tr>
</table></td></tr></tr></tbody></table>

</details>

___

Author: buger

gateway/health_check.go

@@ -187,9 +187,26 @@ func (gw *Gateway) liveCheckHandler(w http.ResponseWriter, r *http.Request) {
 		Details:     checks,
 	}
 
-	failCount, criticalFailure := gw.evaluateHealthChecks(checks)
+	var failCount int
 
-	status, httpStatus := gw.determineHealthStatus(failCount, criticalFailure, len(checks))
+	for _, v := range checks {
+		if v.Status == Fail {
+			failCount++
+		}
+	}
+
+	var status HealthCheckStatus
+
+	switch failCount {
+	case 0:
+		status = Pass
+
+	case len(checks):
+		status = Fail
+
+	default:
+		status = Warn
+	}
 
 	res.Status = status
 
@@ -200,7 +217,7 @@ func (gw *Gateway) liveCheckHandler(w http.ResponseWriter, r *http.Request) {
 		addMascotHeaders(w)
 	}
 
-	w.WriteHeader(httpStatus)
+	w.WriteHeader(http.StatusOK)
 	err := json.NewEncoder(w).Encode(res)
 	if err != nil {
 		mainLog.Warning(fmt.Sprintf("[Liveness] Could not encode response, error: %s", err.Error()))
@@ -259,7 +276,10 @@ func (gw *Gateway) readinessHandler(w http.ResponseWriter, r *http.Request) {
 	}
 
 	w.WriteHeader(http.StatusOK)
-	json.NewEncoder(w).Encode(res)
+	err := json.NewEncoder(w).Encode(res)
+	if err != nil {
+		mainLog.Warning(fmt.Sprintf("[Readiness] Could not encode response, error: %s", err.Error()))
+	}
 }
 
 func (gw *Gateway) determineHealthStatus(failCount int, criticalFailure bool, totalChecks int) (HealthCheckStatus, int) {