test(storage): add fake-server bidi read conformance tests - #7149

Description

This PR introduces 10 end-to-end fake-server integration tests for bidirectional streaming reads (open_object / bidi_read_object) in src/storage/tests/open_object_grpc.rs.

All new tests are encapsulated in a self-contained conformance submodule at the bottom of open_object_grpc.rs to avoid polluting existing tests and helpers.

Test Cases Added

• handle_redirect_error: Verifies handling of mid-read BidiReadObjectRedirectedError gRPC responses, ensuring the client transparently reconnects using the redirected read_handle and routing_token with adjusted read offsets.
• handle_redirect_error_on_open: Verifies handling of BidiReadObjectRedirectedError during initial session open, reconnecting with the updated routing token and completing session initialization.
• handle_redirect_error_max_attempts: Verifies redirect limit enforcement when the server repeatedly returns redirects, confirming the client stops retrying once the attempt budget is exhausted.
• retry_settings_max_attempt: Verifies attempt budget limits under retry settings, confirming that repeated mid-stream failures exhaust resume attempts and fail the reader.
• request_option_verification: Verifies propagation of user project, preconditions (if_generation_match, etc.), and Customer-Supplied Encryption Keys (CSEK AES-256) into gRPC request headers and protobuf fields.
• failed_stream_restart_should_fail_all_pending_reads: Verifies that if an underlying stream breaks and cannot restart (NeverResume), all active and subsequent reads fail immediately.
• retryable_error_while_open: Verifies that opening an object session retries automatically when the initial open encounters a transient error (e.g. Unavailable).
• on_complete_without_data: Verifies handling when the server completes a range (range_end: true) without delivering expected data bytes, ensuring the short read returns an error.
• fast_open_read_session: Verifies fast open by bundling read ranges along with the initial ReadObjectSpec request message.
• non_retryable_error: Verifies non-retryable error classification, confirming permanent errors (NotFound) fail immediately on the first attempt without retrying.

Verification

All tests compile cleanly, adhere to crate formatting, and pass with zero warnings:

cargo fmt -p google-cloud-storage
cargo clippy -p google-cloud-storage --profile test -- -D warnings
cargo test -p google-cloud-storage --test open_object_grpc conformance