fix(brokers): map Robinhood's partially_filled state to OPEN

[eaitbrahim]

What

STATE_TO_PORT_STATUS in packages/keel-broker-robinhood/keel_broker_robinhood/translate.py only covered Robinhood's query-filter state enum (open, canceled, filled, failed, pending). The order-response enum instead carries partially_filled and omits pending. A partially_filled order therefore fell through to_port_status's default and read as PENDING.

partially_filled now maps to OPEN: a partially-filled GTC limit is still resting and working at the venue, so OPEN is the honest status; PENDING reads as not-yet-working. OPEN is already in the port's accepted status vocabulary (see the conformance suite's status assertion).

The map now covers the union of both enums (pending was kept, not removed) since either shape could plausibly arrive in a response.

Unverified spelling

Per #198, no Robinhood order object has ever been observed live, and Robinhood's own docs disagree about which state enum applies to the order-response object. partially_filled's exact spelling is therefore doc-sourced, not confirmed. The added comment says so explicitly and names which of the two documented enums it came from, rather than presenting it as verified.

to_port_status's fallthrough-to-PENDING default is unchanged -- its docstring's reasoning (an unrecognised state means the adapter doesn't know the outcome; FAILED would falsely declare a terminal result) still holds and applies to any future unmapped state.

Latent, not a behaviour fix

Confirmed by reading keel/execution/reconcile.py: it branches only on _FILLED = "FILLED" (line 36) and _DEAD = frozenset({"CANCELLED", "CANCELED", "EXPIRED", "FAILED"}) (line 37). The status != _FILLED check at line 102 treats every non-terminal status identically -- OPEN and PENDING take the exact same "still resting, keep polling" path today. This PR corrects a factual claim about the venue's vocabulary; it does not change behaviour until some future consumer distinguishes OPEN from PENDING.

Closes #226
Refs #198

Test plan

TDD: added test_to_port_status_maps_partially_filled_to_open_not_pending, confirmed red before the fix (to_port_status("partially_filled") returned "PENDING"), green after. Existing test_to_port_status_defaults_an_unknown_state_to_pending_not_failed and test_to_port_status_defaults_none_to_pending pin that the fallthrough default is unchanged and unnarrowed.

• uv run ruff check keel tests packages scripts -- all checks passed
• uv run pytest -q -- 2445 passed, 1 skipped (baseline 2444 passed / 1 skipped + 1 new test; the one skip is the conformance candles probe, suite.py:254, as expected)
• uv run mypy -- Success: no issues found in 219 source files

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