fix(proxy): dynamic pro model fallback with account-aware remapping - #2286

Summary

This PR adds account-aware dynamic model remapping with fallback support for Gemini Pro model variants.

Changes

• token_manager.rs: Add resolve_dynamic_model_for_account() method that detects when an account lacks access to a specific Pro tier (high/low) and automatically falls back to the available tier or preview endpoint
• model_mapping.rs: Add explicit pass-through mappings for Gemini 3.x Pro variants (low/high/preview), with generic aliases routing to the preview entrypoint
• handlers/openai.rs & handlers/gemini.rs: Call resolve_dynamic_model_for_account after account selection to apply per-account model remapping
• mappers/common_utils.rs: Minor adjustment to apply_dynamic_model_remapping for consistency
• docs/model-remapping-logic.md: Add documentation explaining the remapping priority chain

Problem Solved

When users have multiple accounts with different Gemini Pro tier access levels (some with gemini-3.1-pro-high, some with gemini-3.1-pro-low), the proxy would route requests to the wrong tier or fail with 404/400 errors. This PR introduces account-aware fallback logic so the proxy automatically selects the best available model tier for each account.

Remapping Priority Chain

1. Dynamic API deprecation rules (DYNAMIC_MODEL_FORWARDING_RULES)
2. Account-specific capability check → fallback to available tier
3. User custom mapping (custom_mapping)
4. System default mapping (CLAUDE_TO_GEMINI)
5. Pass-through for unknown model IDs

Test plan

• Verify that accounts with only gemini-3.1-pro-high access correctly handle requests for gemini-3.1-pro-low
• Verify that pass-through still works for unknown model IDs
• Verify that gemini-3-pro / gemini-3.1-pro (generic aliases) correctly route to *-preview
• Verify that gemini-3-pro-high / gemini-3-pro-low (concrete IDs) pass through unchanged
• Run cargo test for the proxy module

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