Fix #7490: Replace cert-manager dependency in e2e tests

[ShivamJha2436]

Overview

This PR removes the dependency on cert-manager in the e2e test suite and replaces it with a lightweight in-process certificate generation helper.

Previously, the e2e suite relied on cert-manager CRDs to generate test certificates. Although cert-manager was only used for testing, it was included in go.mod, causing its transitive dependencies (notably gateway-api) to affect Contour’s build.

When cert-manager updated its gateway-api dependency to a newer version, it introduced version conflicts with Contour’s own Gateway API usage, resulting in compilation failures.

What this PR does

• Introduces a local certificate helper (test/e2e/certs.go) that:

  • Generates X.509 certificates using Go’s crypto libraries
  • Tracks issuers in memory
  • Creates TLS Secrets (kubernetes.io/tls) directly
  • Reconciles managed Secrets to preserve behavior like cert rotation

• Refactors e2e tests to use the new helper via the shared framework

• Removes cert-manager usage from e2e tests

• Removes cert-manager dependency from go.mod

• Removes cert-manager installation from test/scripts/make-kind-cluster.sh

• Updates test/scripts/README.md to reflect the new test flow

Why this change

• Avoids dependency leakage from test-only dependencies
• Eliminates gateway-api version conflicts introduced by cert-manager
• Simplifies e2e test setup (no external controller required)
• Improves test determinism and reduces cluster complexity

Behavior parity

The new helper preserves key behaviors previously provided by cert-manager:

• Certificate issuance via issuers
• CA-based signing
• Secret regeneration when deleted (important for rotation-related tests)

Testing

• Ran full e2e test suite locally
• Verified TLS-related tests (backend TLS, mTLS, protocol versions)
• Confirmed no dependency conflicts remain in module graph

Related Issue

Fixes #7490

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.84%. Comparing base (e505630) to head (01e8a96).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7511   +/-   ##
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  Coverage   81.84%   81.84%           
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  Files         130      130           
  Lines       15722    15722           
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  Hits        12868    12868           
  Misses       2571     2571           
  Partials      283      283           

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