fix(voice): enforce browser microphone processing

[JulioMCruz]

Summary

• request browser echo cancellation, noise suppression, and automatic gain control for voice calls
• preserve fail-closed call cleanup when microphone capture cannot be enabled
• add focused coverage for capture options and rejection handling

Validation

• focused voice controller tests: 4 passed
• full test suite: 350 passed
• typecheck: passed
• focused ESLint: passed
• production build: passed with existing local public Firebase configuration loaded without output
• full ESLint: baseline remains failing on unrelated pre-existing findings; no changed-file findings
• diff/credential-pattern scan: clean

Scope

No Chat, A2A, transcript, provider, credential, deployment, or production changes.