fix: honor request-level retry options

[pragnyanramtha]

Summary

• use the merged request HTTP options when deciding whether to retry API calls
• preserve client-level retry behavior while enabling per-request retryOptions for unary and streaming requests
• add unit coverage for request-level retry and request override behavior

Why

request() and requestStream() already merge client and request httpOptions, but apiCall() only read this.clientOptions.httpOptions.retryOptions. As a result, httpOptions.retryOptions supplied for a single request was ignored.

Validation

• npm ci (runs the repository prepare/build script)
• npx prettier src/_api_client.ts test/unit/api_client_test.ts --write
• npx eslint src/_api_client.ts test/unit/api_client_test.ts
• npx tsc && npx jasmine dist/test/unit/api_client_test.js
• npm run unit-test
• git diff --check