tests: pin /fastboot wire format + recv-leftover stress

[widgetii]

Summary

Test-only PR — locks in two integration points that previously rested on real-hardware verification alone.

1. /fastboot binary blob (tests/test_power_rack.py)

RackController.fastboot() packs profile + SPL + agent into a single big-endian binary blob that the pod's C handler in rack/firmware/main/http_api.c parses field by field. Any drift between the two breaks bring-up silently — pod returns 400, the host can't tell why.

Six new TestFastbootWireFormat cases:

Case	Why
test_packs_expected_layout	round-trip every field
test_prestep1_passthrough	optional prestep1 not dropped
test_success_response_returned_verbatim	done/elapsed/markers preserved
test_pod_500_returns_json_body_not_exception	protocol failure (500 + JSON) surfaces as a dict, not an exception — callers need failed_phase / error
test_pod_unreachable_raises_power_controller_error	network-layer errors stay raised
test_realistic_blob_size_within_pod_limit	41 834 B for a typical hi3516ev300 upload, under the pod's 1 MiB cap

Shared helper _parse_fastboot_blob() is the inverse of the host packer; if either side bumps the format, the round-trip test fails loudly.

2. Async recv_packet leftover stress (tests/test_agent_protocol.py)

PR #86 added a per-transport leftover buffer so multi-packet TCP chunks don't drop trailing packets. The existing tests covered the two simplest cases (two/three packets in one chunk). Six stress cases added:

Case	Why
test_frame_split_across_two_reads_recombines	half a packet, then the other half (TCP MTU split)
test_large_stream_50_packets_in_one_chunk	catches off-by-one in leftover slicing
test_recv_response_skips_ready_in_leftover	READY skipping still works when READYs are in leftover, not on the wire
test_per_transport_isolation	two transports must not share leftover state
test_incomplete_frame_in_leftover_blocks_until_timeout	half a frame must wait for the rest, never spuriously return partial
test_corrupt_frame_skipped_then_recovers	bit-flipped CRC mid-stream must be discarded; parser picks up the next valid frame

Test plan

• uv run pytest tests/ -x -v --ignore=tests/fuzz — 480 passed / 2 skipped (was 468)
• uv run ruff check tests/ src/defib/
• uv run mypy src/defib/ --ignore-missing-imports

No production code touched.

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