fix(ts-sdk): validate hosted limit order price

[realfishsam]

Summary

• Add TypeScript hosted buildOrder validation for limit-order price presence and positivity.
• Cover missing/zero limit prices with a regression test that verifies no hosted request is dispatched.

Fixes #1287

Test Plan

• npm test -- --runTestsByPath tests/hosted-dispatch.test.ts -t "positive price" --runInBand (pass; used local untracked generated/src stubs because this checkout cannot regenerate TypeScript OpenAPI output: java is unavailable)
• npm test -- --runTestsByPath tests/hosted-dispatch.test.ts --runInBand (blocked by pre-existing/local generated-stub limitations outside this change: fetchBalance empty stub response and NotSupported instanceof mismatch)
• npm run generate:sdk:typescript --workspace=pmxt-core (blocked locally: java: not found)
• git diff --check

[realfishsam]

CI note: the scoped TypeScript hosted price-validation change is covered by the targeted regression test, but generated-sync checks are currently red from unrelated repo-wide drift on main (Hunch exchange surfacing plus generated client/API reference diffs such as market_id/fee trade fields and source_exchange filters). I did not fold that broad drift into this focused fix PR.

Local checks run:

• npm test -- --runTestsByPath tests/hosted-dispatch.test.ts -t "positive price" --runInBand — PASS (with local untracked generated-client stubs because this checkout cannot regenerate OpenAPI output: java is unavailable)
• git diff --check — PASS

Blocked checks observed on GitHub:

• Verify exchanges reach all consumer SDKs — unrelated missing Hunch exports
• Verify client.ts/client.py/API_REFERENCE.md files are up-to-date — broad generated drift unrelated to SDK drift: Python hosted _hosted_build_order_request validates limit-order price presence and positivity; TypeScript _hostedBuildOrderBody does not #1287