fix(sdk): TcpTransport.send_push raises on OP_ERROR_RESPONSE instead of returning the error envelope

[petrpan26]

Summary

`TcpTransport.send_push` was blindly JSON-decoding the response frame and returning the dict to user code. When the server emitted `OP_ERROR_RESPONSE` (e.g. `invalid_event` on a type mismatch), the error envelope was returned as a regular dict — no exception. Embed mode defaults to TCP, so fire-and-forget pushes silently `/dev/null` on validation failure.

PR #120 documented this and locked the buggy behaviour with `test_type_error_at_push.py`. This PR fixes the bug and flips those tests to assert the correct contract.

Fix in `python/beava/_transport.py`

`send_push` (lines 443-490) now mirrors `send_get`:

• After `read_frame`, check `frame.op != OP_PUSH` (the success-echo opcode — server reuses `OP_PUSH`, not a separate `OP_PUSH_RESPONSE`).
• If `OP_ERROR_RESPONSE`: parse the JSON body with try/except guards and `raise RegistrationError(code=err_body["error"]["code"], message=...)`.
• Fallback: `"unparseable_error"` for bad bytes / `"unexpected_frame"` for missing code.

Docstring expanded with success/error wire shapes and `Raises:` section.

Test flips in `python/tests/test_type_error_at_push.py`

• 5 type-mismatch tests flipped from `_assert_push_error(...)` (which asserted the buggy return-dict shape) to `with pytest.raises(RegistrationError) as exc_info: ...; assert exc_info.value.code == ""`.
• Test #2 (float→int silent accept) unchanged — server still legitimately accepts that case via numeric I64↔F64 compat, returns ack_lsn.
• Added 2 new tests with in-process TCP mock server:
  • `test_push_response_unexpected_opcode_raises` — server replies with bogus opcode → `RegistrationError(code="unexpected_frame")`.
  • `test_push_error_response_with_unparseable_body_raises` — non-JSON error body still raises cleanly (no crash).

Test plan

• Before fix: 6/6 passed by asserting the buggy return shape.
• After fix: 8/8 passed (`pytest python/tests/test_type_error_at_push.py`, 21.26s).
• Re-run against current `main` (`b20d2b83`) — clean.
• `ruff check` clean.