refactor: make set_credentials synchronous in InMemoryContextCredentialStore

[odanilosalve]

Description

Fix multiple SonarQube issues across the client auth and transports modules.

Changes

1. Make set_credentials synchronous (SonarQube S7503)

• src/a2a/client/auth/credentials.py — removed async from InMemoryContextCredentialStore.set_credentials (pure dict operations, no I/O)
• tests/client/test_auth_interceptor.py — removed await from 4 call sites

2. Reduce cognitive complexity in AuthInterceptor.before (SonarQube S3776)

• src/a2a/client/auth/interceptor.py — extracted _apply_credential, _ensure_context, _apply_bearer, _apply_api_key helper methods to reduce nesting

3. Fix transport issues (SonarQube S1763, S112, S3776)

• src/a2a/client/transports/base.py — added # noqa: S1763 for necessary yield in abstract async generators
• src/a2a/client/transports/jsonrpc.py — replaced generic Exception with A2AError in _create_jsonrpc_error return type
• src/a2a/client/transports/rest.py — extracted _is_error_info and _extract_error_info helpers to reduce cognitive complexity in _parse_rest_error

4. Remove redundant exception catch (SonarQube S5713)

• src/a2a/client/transports/rest.py — removed redundant json.JSONDecodeError from except (subclass of ValueError)

Checklist

• Tests pass (195/195)
• Lint and type checks pass
• Follows conventional commits spec