feat: Sync from internal staging.

[chengfeitao-google]

PiperOrigin-RevId: 966780314

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[Sync] Update mobile web build configs, Python agent grounding, and travel mode handling

This update brings the following improvements and fixes:

1. Build Infrastructure

• Self-contained iOS and Android web_build: Updated the export configuration to physically duplicate the shared mobile_core/web_build config files (index.html, tsconfig.json, vite.config.ts, core-shell.ts) directly into client/ios/web_build/ and client/android/web_build/.
• Local core-shell imports: Updated main.ts in both iOS and Android web_build to import A2UICoreShell from ./core-shell instead of the shared parent.
• Dropped client/mobile_core: Removed client/mobile_core from the exported repository to ensure npm install and npm run build work natively inside both client/ios/web_build and client/android/web_build without requiring a monorepo structure.

2. Python Agent & Schemas

• Deterministic Place ID replacement (~40% latency reduction): Updated agent_with_grounding.py to replace synthetic PLACE_ID_FOR_* placeholders with grounded Place IDs using deterministic Python string substitution instead of a second LLM cleanup pass.
• Mandatory travel_mode in DirectionsExtractorSchema: Removed default="driving" from DirectionsExtractorSchema to prevent lightweight models from silently defaulting to driving when travel_mode is omitted.
• Expanded travel_mode synonym normalization: Enriched TRAVEL_MODE_MAP in extractor.py and merger.py to normalize a comprehensive set of synonyms (on foot, subway, auto, bus, train, metro, ferry, light rail, public transit, etc.).
• Updated SKILL.md: Added few-shot examples for all four travel modes (driving, walking, transit, bicycling) in directions-template-response/SKILL.md.

3. Web Components

• GoogleMap travelMode propagation: Fixed google_map.ts to propagate the travelMode property to gmp-route-3d, ensuring walking, transit, and bicycling routes render with the correct travel mode styling and timing.

4. Testing

• Added unit tests for travelMode propagation and edge cases in google_map_test.ts.
• Added unit tests for travel_mode schema validation, synonym normalization, and merger in test_extractor.py, test_merger.py, and test_agent_with_templates.py.