chore: remove unused Bedrock auth lazy loading (#18948)

## 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`

codex-rs/Cargo.lock

@@ -2891,7 +2891,6 @@ dependencies = [
  "codex-protocol",
  "http 1.4.0",
  "pretty_assertions",
- "tokio",
 ]
 
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