ci: use self-hosted httpbun in tests

[AlexV525]

Motivation

httpbun.com is currently returning HTTP 502 responses, which makes the integration tests fail independently of dio changes. The failure is visible in the PR #2567 stable job. CI already starts httpbun.local, but HTTP/2 pinning and all browser tests still used the public service.

upstream sharat87/httpbun#35

Changes

• Reuse the shared test-server URL in HTTP/2 pinning tests and generate their certificate fingerprint from the configured host.
• Keep local HTTPS for VM and HTTP/2 tests.
• Start a second local HTTP container for Chrome, Firefox, and Flutter tests, avoiding browser-specific temporary-CA trust configuration.
• Keep httpbun.com as the default outside the CI-local setup.
• Avoid assuming onBadCertificate and validateCertificate receive different certificates; that varies with the certificate chain and trust store.

This touches TLS and certificate-pinning test infrastructure only; production TLS behavior is unchanged. Maintainers: please apply the extra scrutiny required for this sensitive area.

Verification

melos run format and melos run analyze pass. The local Chrome shared suite passes 54 tests with one existing skip against the HTTP container, including CORS, redirects, status codes, timeouts, and uploads. GitHub Actions passes on the minimum, stable, and beta SDK jobs, covering VM, Chrome, Firefox, and Flutter tests; the stable job also passes APK build and coverage generation.

Implementation, tests, and local review by Codex.

[github-actions]

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
plugins/web_adapter/lib/src/cors.dart	🟢 100%	🟢 96.55%	🔴 -3.45%
Overall Coverage	🟢 85.73%	🟢 85.68%	🔴 -0.05%

Minimum allowed coverage is 0%, this run produced 85.68%