fix: initialize Probable order book resolver queues

[iccccccccccccc]

Fixes #264

Changes

• Replace the guarded orderBookResolvers.get(...)! push path with an explicit queue initializer.
• Add a regression test so Probable websocket resolver queueing does not reintroduce the non-null assertion pattern.
• Include the current generator fixes from fix: initialize Baozi order book resolver queues #509 so this PR passes the existing exchange/SDK drift gates while main is still out of sync.

Verification

• npm --workspace=pmxt-core test -- --runTestsByPath test/pipeline/probable-websocket-resolvers.test.ts --runInBand
• npx tsc -p core/tsconfig.json --noEmit
• node core/scripts/check-exchange-drift.js
• node sdks/typescript/scripts/generate-client-methods.js; git diff --exit-code sdks/typescript/pmxt/client.ts
• node sdks/python/scripts/generate-client-methods.js; git diff --exit-code sdks/python/pmxt/client.py
• python -m py_compile sdks/python/pmxt/_exchanges.py sdks/python/pmxt/__init__.py sdks/python/pmxt/client.py
• git diff --check

[realfishsam]

PR Review: PASS (NOT VERIFIED)

What This Does

Fixes a non-null assertion bug in Probable's WebSocket order book resolver queueing (issue #264) by extracting queue initialization into a getOrderBookQueue() helper. Also carries forward two generator fixes from #509: Python SDK class naming for underscore-delimited exchanges (Polymarket_us -> PolymarketUS), and hardcoded fetchOrderBook special-case generation in both SDK client generators.

Blast Radius

• Core: probable/websocket.ts only -- no other exchanges touched
• Python SDK: _exchanges.py, __init__.py, generate-python-exchanges.js -- class name change is a breaking change for Python consumers using pmxt.Polymarket_us
• Both SDKs: generate-client-methods.js scripts -- adds fetchOrderBook special-case override to generators (output matches what is already in the committed client files)
• Docs: API_REFERENCE.md in both SDKs -- updated to reflect FetchOrderBookParams type and close method (replacing stale testDummyMethod reference)

Test Results

• Build: NOT RUN (worktree has no node_modules, npm install was blocked by permissions)
• Unit tests: NOT RUN locally
• CI checks: ALL PASS (6/6 -- exchange drift, SDK sync, API reference, compliance)

Findings

1. Polymarket_us -> PolymarketUS is a breaking change for Python consumers (sdks/python/pmxt/_exchanges.py:368, __init__.py:20). Any user doing import pmxt; client = pmxt.Polymarket_us() will get an ImportError. The exchange_name="polymarket_us" wire string is unchanged so the sidecar still works, but the public API surface changed. This may warrant a deprecation alias or a semver minor/major bump depending on project policy. Note: the old name Polymarket_us is not Pythonic (PEP 8), so the rename to PolymarketUS is correct -- but it is still a breaking symbol rename.

2. The test is a source-text regex guard, not a behavioral test (core/test/pipeline/probable-websocket-resolvers.test.ts:5-12). It asserts that the pattern orderBookResolvers.get(...)! does not appear in the source file. This prevents regression of the specific anti-pattern but does not verify the runtime fix. Acceptable for a regression guard, but worth noting.

3. Seven other websocket files have the identical unsafe pattern (baozi, opinion, kalshi [x2], myriad, polymarket, gemini-titan). These are out of scope for this issue ([non-null] probable/websocket.ts: unsafe Map.get()! assertion on orderBookResolvers #264 is Probable-specific), but the same Map.get()! bug exists there. This is informational, not a blocker.

4. The fetchOrderBook generator special-case duplicates existing hand-maintained logic (sdks/python/scripts/generate-client-methods.js:390-422, sdks/typescript/scripts/generate-client-methods.js:384-419). The generated output matches what is already committed in client.py and client.ts, so this is a no-op change that ensures future regeneration produces the same output. No issues found.

5. The toClassName regex change (core/scripts/generate-python-exchanges.js:33) splits on both - and _, and special-cases us -> US. This correctly handles polymarket_us -> PolymarketUS, kalshi-demo -> KalshiDemo, and gemini-titan -> GeminiTitan. No other exchange names contain underscores, so the blast radius is limited to polymarket_us.

PMXT Pipeline Check

• Field propagation (3-layer): N/A (no new fields)
• OpenAPI sync: N/A (no BaseExchange.ts changes)
• Type safety: OK (non-null assertion removed, no any introduced)

Semver Impact

minor (or patch depending on policy) -- the core fix is patch-level, but the Python class rename Polymarket_us -> PolymarketUS is a breaking symbol change for Python SDK consumers. If any users reference Polymarket_us by name, they will break on upgrade. If the project treats SDK class names as part of the public API, this is technically a major change -- but given that Polymarket_us was likely a bug in the first place (non-PEP-8), treating it as minor with a changelog note seems reasonable.

Risk

• Local build and tests were not run (CI passed all 6 checks)
• The Python class rename has no deprecation alias -- existing pmxt.Polymarket_us users will break silently on upgrade
• The 7 other websocket files with the same Map.get()! pattern remain unfixed