chore: release 3.19.0

[shreemaan-abhishek]

Description

Release 3.19.0.

This PR:

• Adds the 3.19.0 section to CHANGELOG.md (and its Table of Contents entry), covering all PRs merged since the release/3.18 branch was cut. Backward-incompatible changes are grouped under Change and marked with ⚠️.
• Bumps the version to 3.19.0 in apisix/core/version.lua and docs/{en,zh}/latest/config.json.
• Extends ci/check_changelog_prs.ts IGNORE_PRS with changes that are not user-facing: CI-only work whose fix(ci) subject prefix dodges the docs/chore/test/ci type filter (fix(ci): unbreak CI under pnpm 12 #13921, fix(ci): stop rerun_flaky_tests from turning unparsable failures into a pass #13923).
• Bumps APISIX_DASHBOARD_COMMIT in .requirements to fa2fd0f (current apisix-dashboard master). APISIX_RUNTIME is already at 1.3.18, so it is unchanged.

Breaking changes (see the Change section)

Only changes that break an existing configuration or stored state on upgrade, with no adjustment made, are listed here.

• feat(upstream): verify the upstream certificate against configurable CAs #13863 feat(upstream): verify the upstream certificate against configurable CAs
• fix(openid-connect): validate introspection issuer #13916 fix(openid-connect): validate the introspection issuer
• fix(batch-requests): bound aggregated response bodies #13906 fix(batch-requests): bound aggregated response bodies
• fix(basic-auth): reject an empty consumer password #13884 fix(basic-auth): reject an empty consumer password
• fix(ai-proxy-multi): reject instances that share a name #13851 fix(ai-proxy-multi): reject instances that share a name
• fix(workflow): reject invalid case expressions and missing action conf #13862 fix(workflow): reject invalid case expressions and missing action conf

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change (N/A: release bookkeeping only, no runtime code changes)
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (this PR itself only touches release metadata; the breaking changes it documents landed in their own PRs and are listed in the Change section)

[shreemaan-abhishek]

Superseded: the release commit is pushed to release/3.19 directly and the PR now targets master, matching the 3.18.0 release flow (#13842).