fix: accept disabled failover aliases

[1jehuang]

Summary

• accept off, false, disabled, and none as aliases for manual cross-provider failover
• keep the existing countdown default and serialization unchanged
• cover both TOML deserialization and the environment-value parser

Verification

• alias regression test passes for all four values
• default-behavior regression test passes
• jcode-config-types and jcode-base check successfully

Fixes #1101

--- — Jcode agent (automated triage), on behalf of @1jehuang

[greptile-apps]

Greptile Summary

This change expands the accepted disabled-value aliases for cross-provider failover while preserving manual as the normalized configuration value. Configuration loading was exercised with off, false, disabled, and none through both TOML and JCODE_CROSS_PROVIDER_FAILOVER; each value resolved to manual failover behavior.

Confidence Score: 5/5

Safe to merge based on successful public configuration loading checks across both supported input paths.

The focused regression test exercised all four new aliases in TOML and environment overrides, and confirmed the updated behavior resolves each alias to manual failover.

Files Needing Attention: No files need further attention.

T-Rex Logs

What T-Rex did

• I ran the authored public-configuration loading test against the base commit and the updated commit using cargo test with the specified flags, and observed that the base commit rejected cross_provider_failover = "off" as an unknown variant and exited 101 while the updated commit passed with 1 passing test and 0 failures, loading all four aliases through both TOML and JCODE_CROSS_PROVIDER_FAILOVER and asserting them as Manual.
• I reviewed the post-merge test results and confirmed that after the PR change the test run completed successfully: 1 passed, 0 failed, exit 0, and all eight cases (four aliases across two mechanisms) resulted in Manual across both TOML and environment-based loading.

_{Ran code and verified through T-Rex}

_{Reviews (1): Last reviewed commit: "fix: accept disabled failover aliases (f..." | Re-trigger Greptile}