fix: let fetch calculate request content length

[shrey150]

Why

The SDK currently calculates Content-Length before handing a request to the configured Fetch implementation. That value becomes stale if Fetch middleware transforms the body afterward, and Node's native Fetch then rejects the request with UND_ERR_REQ_CONTENT_LENGTH_MISMATCH.

The ordinary session-create JSON path is byte-correct on its own. The compatibility problem is ownership: Fetch already calculates request framing from the final body, and current Stainless-generated TypeScript clients leave that work to the transport.

What changed

• stop synthesizing content-length in the SDK request builder
• preserve an explicitly caller-provided value
• add request-builder and real HTTP wire coverage for native Fetch and the node-fetch fallback

Root cause

The length calculation itself predates the reported regression. SDK 2.14.1 changed modern Node runtimes from node-fetch to native globalThis.fetch; native Fetch/Undici strictly validates an explicit length against the final bytes. A body-transforming wrapper can therefore expose a stale header that the earlier transport path did not surface. Delegating the header to Fetch makes it calculate the value after those transformations.

E2E Test Matrix

Command / flow	Observed output	Confidence / sufficiency
node@22 <harness> local minify	Middleware changed JSON from 217 to 159 bytes; the SDK supplied no length; native Fetch sent Content-Length: 159; server received 159 bytes; PASS.	Reproduces the previously failing stale-header shape and proves the fix on Node 22/Undici 6.
node@24 <harness> local minify	Middleware changed JSON from 217 to 159 bytes; the SDK supplied no length; native Fetch sent Content-Length: 159; server received 159 bytes; PASS.	Proves the same regression path on Node 24/Undici 7.
node@22 <harness> local node-fetch and node@24 <harness> local node-fetch	In both runs, node-fetch generated Content-Length: 217; server received 217 bytes; PASS.	Covers the fallback transport used on older runtimes and when callers provide a Node HTTP agent.
node@22 jest tests/content-length.test.ts and node@24 jest tests/content-length.test.ts	Three wire-level tests passed on each runtime: native Fetch, the node-fetch fallback, and body-transforming middleware.	Keeps the transport behavior and the reproduced regression covered in the repository.
node@22 <local Stagehand 3.7 flow> and node@24 <local Stagehand 3.7 flow>	Real Browserbase stagehand.init() created a session and close() released it on both runtimes; PASS.	Exercises the exact downstream Stagehand initialization path with the local SDK build and real service.
node@22 <local SDK flow> and node@24 <local SDK flow>	Real Browserbase session create and release succeeded on both runtimes; SDK supplied no explicit length; PASS.	Exercises the changed SDK directly against the live API; no live resource identifiers are included.
node@20 jest --runInBand with the generated mock API	22 suites, 140 tests, and 2 snapshots passed.	Full repository regression suite.
npm run build on Node 20	CommonJS and ESM builds completed with 0 TypeScript errors; both package import smoke checks passed.	Proves the exact branch produces publishable artifacts in the repository's supported build environment.
npm run lint	ESLint and TypeScript checks passed.	Supporting static verification.