Remove HTTP-era leftovers from the WebSocket coordinator

[hermabr]

Stacked on #470. Follow-up simplification: the transport migrated to WebSockets, but three HTTP-era concepts survived in the new code. This deletes them (−94 production lines net, plus one fewer race and two fewer protocol concepts).

Changes

1. Blocking lease_job on a single Condition. "wait" existed because HTTP couldn't block a request. lease_job now blocks until a job is available or the run is done (Job | None), the lock/wake pair merges into one threading.Condition notified by mutators inside the locked section, and the server's 1s wake poll disappears — along with the missed-wakeup race between lease_job releasing the lock and the server thread acquiring wake. Tests that asserted "wait" now assert via count_satisfiable_jobs (same predicate, non-blocking).
2. Drop redundant message envelopes. The worker→server channel is strictly sequential: one hello, then exactly one result per assignment — there is never a point where the server could receive either. Hellos validate directly with HelloMessage.model_validate_json, result frames are a bare JobResult (already discriminated on status; the adapter moves from execute.py to protocol.py), and ResultMessage, worker_message_adapter, and HelloMessage.kind go away. AssignMessage | StopMessage stays — that direction is a genuine union.
3. lease_job no longer releases prior leases. Release-on-re-lease was load-bearing under HTTP (a crashed worker could simply request again). Over the socket the server thread enforces the sequence — it never re-leases while results are outstanding — so connection teardown (worker_lost) is now the single release path, and lease_job is side-effect-free on other state.
4. Fold client.py into worker_loop. One caller, one connect() + one send(), and an always-true isinstance assert.

Explicitly considered and rejected: an explicit protocol state machine (the thread-per-connection blocking design already encodes the states in control flow), in-process execution for the local backend, and event-driven pool scaling.

Testing

• pytest tests/test_worker_execution_coordinator.py tests/test_worker_subprocess.py tests/test_slurm_backend.py: 127 passed.
• Full suite: 609 passed; the 3 failures in test_furu_locking_contention.py reproduce identically without this change (deadline-sensitive tests on AFS) and are unrelated.
• ruff check, ruff format, ty check: clean.

[hermabr]

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• Remove HTTP-era leftovers from the WebSocket coordinator #475 👈 (View in Graphite)
• Migrate execution coordination from HTTP to WebSockets #470 : 2 other dependent PRs (#476 , #481 )
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