chore: remove unused Bedrock auth lazy loading

[celia-oai]

Summary

The Bedrock Mantle SigV4 auth provider currently looks like it can lazily load AwsAuthContext, but the provider is only constructed after resolve_auth_method has already loaded that context. Because with_context always pre-populates the OnceCell, the get_or_try_init fallback is unused in normal operation and makes the provider lifecycle harder to reason about.

This change removes that dead lazy-loading path and makes the actual behavior explicit:

• BedrockAuthMethod::AwsSdkAuth carries only the resolved AwsAuthContext.
• BedrockMantleSigV4AuthProvider stores the resolved context directly.
• request signing uses the stored context without going through OnceCell.

The existing eager AWS auth resolution behavior is unchanged; this is a simplification of the provider state, not a behavior change.

Testing

• cargo shear
• cargo test -p codex-model-provider
• just bazel-lock-check