Fix WebSocketMessage type contract

[pengpengyi92]

Summary

• define WebSocketMessage as the union of individual parsed event models instead of a new type over a list
• keep batch callbacks typed as List[WebSocketMessage], matching the runtime parser output
• add runtime and mypy regression coverage for single-event and batch contracts

Root cause

parse_single() returns one event model, while parse() returns a list. The previous alias represented a list itself, so callback annotations became a nested List[WebSocketMessage] and parse_single() needed an unsafe cast to hide the mismatch.

Validation

• python -m unittest discover -s test_websocket (3 passed)
• mypy massive test_websocket (50 files, no issues)
• black --check massive test_websocket
• git diff --check

The REST suite could not start locally because its longest fixture path is corrupted by the Windows non-ASCII workspace path; all failures occurred while loading the same unrelated fixture before test execution. Upstream CI should provide the full Linux run.

Closes #1022