[TexasCoding]

Closes #550

Summary

Kalshi added the public perps endpoint GET /margin/corporate_actions, which made test_every_spec_endpoint_is_mapped[perps] fail in the nightly spec-drift run.

• Re-synced all vendored specs (scripts/sync_spec.py) and regenerated kalshi/_generated (scripts/generate.py, no hand edits).
• New perps.margin.corporate_actions(*, ticker=None, market_id=None) sync + async. Public (no auth), per the spec.
• New Pydantic models: GetMarginCorporateActionsResponse, MarginCorporateAction, and cash / split / final_settlement / ticker_change terms. FixedPointDollars amounts are DollarDecimal. Terms and execution_time are optional because cancelled notices leave them out.
• Registered in kalshi/_contract_map.py and tests/_contract_support.py.
• PerpsChannel.CORPORATE_ACTIONS added so the WS channel enum matches the re-synced perps AsyncAPI.
• Tests: happy path, 400/500 errors, cancelled notice with no terms, empty list, query filters, sync + async.
• Version 22.2.0 (additive, minor). CHANGELOG and docs/perps.md updated.

Blast radius (hand-written; GitNexus unavailable)

• kalshi/perps/models/margin_account.py, kalshi/perps/resources/margin_account.py: new code only; existing methods unchanged.
• kalshi/perps/__init__.py, kalshi/perps/models/__init__.py: new exports only.
• kalshi/perps/ws/models/control.py, kalshi/perps/ws/client.py: one new enum member; existing subscriptions unchanged.
• Contract maps, tests, docs, specs, version files.
• No breaking changes for existing callers.

Gates

• ruff check .: pass
• mypy kalshi/: pass
• pytest tests/ --ignore=tests/integration -q: 4757 passed, 3 skipped
• pytest tests/test_contracts.py -W error::UserWarning: 864 passed