chore: restore release safety baseline

[Fiona2016]

Motivation

Restore a reliable green CI/release baseline for the Flashcat browser SDK. The previous release path relied too much on manual local checks, and e2e tests still had several local/offline reliability issues, including the e2e intake server forwarding requests to browser-intake-datadoghq.com during tests.

Release-flow recommendation from this work:

• PR / release preflight should run the full safety baseline: install, format, lint, typecheck, build, unit, package checks, compatibility, release bundle, script tests, and e2e.
• The actual publish job should not be the first place that discovers long-running e2e/build failures. It should mainly validate immutable inputs, use artifacts produced by a verified preflight, and run quick smoke/metadata checks before publishing.
• Relying on "local full run + publish only unit tests" is not safe enough: local state is not reproducible, and release safety should be enforced by GitHub Actions on the exact commit being released.

Changes

• Add a GitHub Actions CI baseline for PR/push checks.
• Harden deploy workflows with explicit release preflight, immutable install, manual confirmation, artifact handoff, and safer version handling.
• Add/adjust deploy script tests, including OSS prerelease detection.
• Restore deterministic local e2e behavior:
  • test:e2e:ci initializes test apps before running e2e.
  • e2e intake no longer forwards requests to browser-intake-datadoghq.com.
  • non-BrowserStack e2e servers use OS-assigned free ports to avoid worker port collisions.
  • flaky session flush / tracing / resource / view assertions were tightened.
• Fix compatibility-test app package resolution and make compatibility cleanup non-destructive to the working tree.

Test instructions

Fresh local validation on this branch:

• yarn install --immutable passes with existing peer warning: performances@workspace:performances does not provide typescript requested by ts-node.
• yarn format
• yarn lint
• yarn typecheck
• scripts/cli typecheck test/e2e
• yarn build
• node scripts/check-packages.js
• yarn test:unit --single-run -> 2643 SUCCESS
• yarn test:compat:tsc
• yarn test:compat:ssr
• BUILD_MODE=release yarn build:bundle
• yarn test:script
• yarn test:e2e:ci -> 444 passed, no flaky/retry in the final run

Targeted flaky regression checks:

• forward logs to the bridge --repeat-each=5 -> 15 passed
• flush events when the session expires --repeat-each=10 -> 30 passed
• getScrollX/Y should not be affected by pinch scroll --repeat-each=10 -> 30 passed

Also verified e2e code no longer contains browser-intake-datadoghq, forwardIntakeRequestToDatadog, E2E_FORWARD_INTAKE, or e2e https.request forwarding.

Checklist

• Tested locally
• Tested on staging
• Added unit tests for this change.
• Added e2e/integration tests for this change.