#30775 fix(acpmux): register Cloud turn methods and TLS provider

Fixes the two own reds exposed by merged Cloud turn rewind #30337.

• Register agent-session-turn-prepare and agent-session-turn-abandon in the acpmux protocol registry so served-method and capability coverage checks agree with the dispatcher.
• Install rustls' ring CryptoProvider before constructing the Cloud turn reqwest client, preventing the reqwest 0.13 No rustls crypto provider configured panic in direct startup/test paths.

Validation: targeted protocol and Cloud turn tests plus the acpmux library suite via cmux-ci; the remote-guard missing-home fixture failure is base/environment-only and is not changed here.

Agent session: agent session

---

^{Need help on this PR? Tag @codesmith-bot with what you need.}

---

Summary by cubic

Fixes the regressions exposed by the Cloud turn rewind merge and refreshes the API compatibility surface.

• acpmux now registers agent-session-turn-prepare and agent-session-turn-abandon so served-method and capability checks agree with the dispatcher.
• The Cloud turn client installs rustls' ring CryptoProvider before building its reqwest client, preventing the "no rustls crypto provider configured" panic in direct startup and test paths. Installing the provider is idempotent when another path already configured one.
• Refreshes api-compat.surface.json with cloud-environment request and response types.