feat(rpc): support multiple API keys

[raushan728]

Migrates api_key: Option<String> to api_keys: Option<Vec<String>>, with backward-compatible deserialization for existing single-key configs (with a deprecation warning). Compares keys via fixed-length SHA-256 hashes with constant-time comparison across all configured keys, avoiding any length-dependent timing signal, and stores a key index as the client identity.

Note: this is a split-out of #559.

[greptile-apps]

Greptile Summary

Adds support for multiple configured API keys while retaining backward-compatible deserialization of the legacy single-key setting.

• Resolves API keys from configuration or the environment and warns when environment configuration overrides multiple keys.
• Authenticates against every configured key using fixed-size SHA-256 digests and assigns the matched key index as the client identity.
• Updates validation, test utilities, fixtures, and example configurations for the new array format.
• The prior key-length timing finding is fixed: comparisons now operate on fixed-size digests, and the authentication loop does not branch on supplied-key length or exit after a match.

Confidence Score: 5/5

The PR appears safe to merge, with the previous timing concern addressed and no new actionable failures identified.

API-key authentication now hashes both sides to fixed-size digests, checks every configured key without early exit, and retains correct matching behavior for keys of different lengths. No repository-rule violations or outstanding previous findings remain.

Important Files Changed

Filename	Overview
crates/lib/src/rpc_server/auth.rs	Replaces variable-length key comparisons with fixed-size digest comparisons across all configured keys and records the matched key index.
crates/lib/src/config.rs	Migrates authentication configuration to multiple keys with legacy single-key deserialization and environment override handling.
crates/lib/src/rpc_server/server.rs	Constructs API-key middleware from the resolved list of configured keys.
crates/lib/src/validator/config_validator.rs	Adapts configuration warnings to the multi-key environment override behavior.

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