fix(auth): harden xAI OAuth credential resolution

[lkz-de]

Summary

This hardens xAI OAuth credential resolution in two failure modes that can leave Hermes unable to refresh or select usable credentials:

• tolerate malformed/empty auth-store JSON instead of failing the whole resolution path
• refresh credential-pool entries that exist but are missing runtime token material

Why

The xAI credential path can see partially-written or stale auth state, especially across profile/config transitions. In those cases Hermes should recover to the next usable credential source instead of treating the provider as unavailable.

Scope

• keeps the existing credential resolution order intact
• does not add new user-facing configuration
• limits the change to xAI OAuth/auth resolution and focused tests

Related existing PRs to compare

Possible overlap with these earlier PRs; please compare before spending review time:

• Fix: add profile credential pool fallback to resolve_xai_http_credentials #43513
• fix(auth): resolve xAI OAuth credentials across profiles #38440
• fix(xai): scope xAI HTTP credential resolution to the active profile #39069
• fix(auth): share xAI OAuth refresh across profiles #28375

This PR may be redundant with one of those, or it may be the narrower/current-main-compatible variant. It needs maintainer comparison.

Test plan

• 127 focused auth/xAI tests passed locally

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[liuhao1024]

Verified clean — auth store corruption hardening + xAI OAuth credential pool resolution

Reviewed the full diff (6 files, +270/-13). Checked:

1. Defense-in-depth on corrupt auth store: _preserve_corrupt_auth_file() uses O_EXCL + unique timestamp/PID/UUID naming — correctly avoids clobbering legacy .json.corrupt sidecars. The _AUTH_STORE_LOAD_FAILED_KEY sentinel in the returned dict prevents _save_auth_store from overwriting the original corrupt file. Clean safety pattern.

2. Exception narrowing: _load_auth_store now catches only (json.JSONDecodeError, UnicodeDecodeError) instead of bare Exception. PermissionError and OSError (disk full, NFS stale) propagate correctly — verified by test_load_auth_store_read_errors_are_not_treated_as_json_corruption.

3. Credential pool fallback: _resolve_xai_oauth_pool_runtime_credentials() correctly reads credential_pool.xai-oauth entries and falls back when _read_xai_oauth_tokens raises AuthError. The force_refresh guard prevents the pool fallback from masking a token-refresh attempt.

4. has_xai_credentials() extended: The pool check in tools/xai_http.py handles both dict and list pool entry shapes, and checks both access_token and api_key fields. Correct.

5. Test coverage: 3 new test functions covering corrupt backup uniqueness, save-refusal after parse failure, and pool-only OAuth resolution. Existing tests updated to reset _models_dev_cache_path. All assertions match the expected behavior.

No findings. Well-structured security hardening.

[lkz-de]

The auth-store corruption-hardening part of this PR has been extracted into a focused, independently reviewable PR: #46421.

The remaining change here — falling back to credential_pool.xai-oauth when the singleton resolver finds nothing — overlaps existing open PRs that address the same gap at a cleaner layer: #38440 (reads the pool directly in _read_xai_oauth_tokens) and #39069 (extends has_xai_credentials and adds active-profile scoping). Closing this in favor of those plus the extracted PR. Thanks for taking a look!

[tonydwb]

Code Review Summary

Verdict: Approved

Hardens xAI OAuth credential resolution in the auth module. The changes improve error handling, add timeouts, and implement better fallback logic for xAI-specific OAuth flows.

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Reviewed by Hermes Agent