feat(health): add /healthz and /readyz endpoints

[fseldow]

Summary

• add unauthenticated /healthz and /readyz probe handlers
• track server liveness/readiness and mark readiness once an executor is available
• add focused unit coverage for health endpoint responses

Testing

• go test ./internal/httpserver

Unified health probe

• add a dedicated plain HTTP health probe server on :9090 for /healthz and /readyz
• register readiness checks for both the controller-runtime manager and the Ratify HTTP server executor
• wire the --health-port flag and Helm chart values to use the unified probe port

Addresses #2562

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds Kubernetes-style liveness/readiness endpoints to the internal HTTP server and verifies their behavior with unit tests.

Changes:

• Introduces /healthz (liveness) and /readyz (readiness) handlers and routes.
• Adds a background readiness probe that marks the server ready once an executor is available.
• Adds unit tests covering health and readiness handler responses.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
internal/httpserver/server.go	Adds health state to the server, registers health routes, and starts readiness polling.
internal/httpserver/health.go	Implements JSON health/readiness handlers backed by atomic state.
internal/httpserver/health_test.go	Adds unit tests for /healthz and /readyz responses.

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[codecov]

Codecov Report

❌ Patch coverage is 85.04983% with 45 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.16%. Comparing base (c6a515a) to head (48e03b5).

Files with missing lines	Patch %	Lines
internal/manager/manager.go	0.00%	16 Missing ⚠️
internal/httpserver/server.go	37.50%	11 Missing and 4 partials ⚠️
cmd/ratify-gatekeeper-provider/main.go	42.85%	7 Missing and 5 partials ⚠️
internal/manager/ready.go	92.85%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2559      +/-   ##
==========================================
+ Coverage   77.62%   78.16%   +0.54%     
==========================================
  Files         105      109       +4     
  Lines        4657     4956     +299     
==========================================
+ Hits         3615     3874     +259     
- Misses        893      923      +30     
- Partials      149      159      +10     

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