Resolve RPC auth follow-up comments

[YellowSnnowmann]

Resolved the remaining follow-up comments from the PR review:

1. Auth Middleware Tightening: Updated rpc_auth_middleware to use an explicit (method, path) allowlist, ensuring only GET requests to public paths and OPTIONS requests are bypassed.
2. Documentation Accuracy: Corrected the module-level documentation in src/core/auth.rs to accurately reflect how the Tauri shell and frontend handle the RPC token.
3. Frontend Robustness: Implemented a single-retry mechanism with a 100ms delay in coreRpcClient.ts when fetching the token via the core_rpc_token Tauri command.
4. Test Safety: Addressed potential unsoundness in tests by moving std::env::set_var into one-time initialization closures, wrapping them in unsafe blocks, and ensuring they are serialized by existing environment locks.
5. Observability: Added a success log message upon successful bearer token initialization in the core server.
6. Maintainability: Added comments explaining manual header construction in the core updater and improved the robustness of init_rpc_token.

All tests in json_rpc_e2e and core auth unit tests passed.

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PR created automatically by Jules for task 15793745111021328236 started by @YellowSnnowmann

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