feat: add batch order helpers

[harley-poly]

Changes

Add orders.create_many(), cancel_many() and modify_many() to both Python clients, with typed requests/responses and usage examples. Each helper sends one authenticated request through the existing transport, preserving its no-retry policy for orders.

Document the 20-order limit and that cancel/modify response IDs confirm submission, not individual order outcomes. Accept fractional quantities in the reused create/modify request types. These two annotations overlap #6; its response quantity changes remain separate.

Set the additive SDK version to 2.4.0, to release after #15's 2.3.0. Follows the SDK follow-up in https://github.com/Polymarket/go-gateway-us/pull/751.

Testing

• 271 tests pass on Python 3.10 and 3.13, including 48 new batch cases covering signatures, fractional/whole quantities, payloads, responses, authentication, API errors/timeouts and no retries or splitting.
• Typed consumers exercise all new methods and exported types, including fractional and whole quantities.
• Ruff lint/format, mypy and wheel/sdist build pass. No live orders submitted.

Checklist

• Tests pass (pytest)
• Linting passes (ruff check .)
• Types check (mypy polymarket_us)

---

Note

Medium Risk
New trading endpoints with non-idempotent POST semantics; incorrect client assumptions about cancel/modify response IDs could cause operational mistakes, though behavior matches existing single-order retry policy.

Overview
Adds batch order APIs to the sync and async clients: orders.create_many, cancel_many, and modify_many, each posting up to 20 items in one authenticated request to /v1/orders/batched (and /cancel, /modify). New TypedDicts cover batch request/response shapes; create/modify quantity is widened to int | float for fractional sizes.

README documents the 20-order cap, semantics of returned ID lists (create returns exchange IDs; cancel/modify echo submitted IDs without per-order confirmation), and that batches are not auto-retried or split—same as single-order POSTs. Package version bumps to 2.4.0; tests cover signing, payloads, errors, timeouts, auth, and server-side batch size validation.

^{Reviewed by Cursor Bugbot for commit a407ed7. Bugbot is set up for automated code reviews on this repo. Configure here.}