feat: Add optional streaming members to the platform fetch types

[joker23]

Summary

Extends the platform Requests types in @launchdarkly/js-sdk-common so a platform fetch implementation can serve as the SSE streaming transport for @launchdarkly/eventsource through structural typing, with no dependency between the two packages in either direction.

New members on the platform types, all optional so existing platform implementations remain valid and external implementers see no breaking change:

• Options.streaming -- marks a streaming request: the implementation does not request compressed content, applies no request timeout, and delivers response chunks with low latency through the response body.
• Options.signal -- an AbortSignalLike abort signal. The type resolves to the platform's own global AbortSignal when one is declared, and falls back to a minimal structural subset otherwise, which keeps the published declarations valid for consumers that compile without DOM or Node global types.
• Response.statusText and Response.body (with the new ResponseBody and BodyReader interfaces) -- the streamable response surface.
• Headers.forEach -- the header iteration the streaming client reads.

On the eventsource side, FetchLikeResponse.statusText and FetchHeaders.forEach become optional, so the contract -- Requests['fetch'] is assignable to FetchLike -- holds at the interface level. The client already guarded a missing statusText at runtime; headersToObject gains the matching guard for a missing forEach. Compile-level assertions in the eventsource type-compatibility tests enforce the contract and pin its two deliberate exceptions (credentials and url). Note for external consumers of @launchdarkly/eventsource: the read type of statusText widens to string | undefined, and FetchHeaders.forEach becomes optional, so code that reads these members directly must guard them.

The Shopify Oxygen implementation converts the platform options at its native fetch boundary, because its test configuration declares RequestInit without a global AbortSignal constructor.

No runtime behavior changes beyond the header guard: a transport that supplies no header iteration now reports an empty header set instead of failing the stream as unreadable. This is a deliberate decision, and it follows the existing rule that a 200 response with no Content-Type header is accepted, because a minimal transport can report no headers. The cost is that a transport which forgets forEach loses Content-Type validation on a 200 rather than failing loudly.

This is the bottom of a two-PR stack. #2060 moves the node-family SDKs' streaming transports onto this seam and deletes their parallel fetch adapters.

---

Note

Overview
Extends the shared platform Requests/Options/Response types so a platform fetch can act as the SSE transport for @launchdarkly/eventsource without a package dependency: optional streaming, signal (AbortSignalLike), response body/BodyReader, optional statusText, and optional Headers.forEach.

On the eventsource side, FetchHeaders.forEach and statusText are optional to match; headersToObject now treats missing forEach as an empty header set (200 streams can open without Content-Type validation). Compile-time checks in typeCompatibility.test.ts lock Requests['fetch'] ↔ FetchLike assignability (with documented exceptions for credentials and url).

Adds internal.toRequestInit and wires Shopify Oxygen to pass platform options through it at native fetch/Request boundaries for AbortSignal typing. New tests cover transports without header iteration or statusText.

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[github-actions]

@launchdarkly/js-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 30631 bytes
Compressed size limit: 30000
Uncompressed size: 145908 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 25437 bytes
Compressed size limit: 44000
Uncompressed size: 165420 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 33022 bytes
Compressed size limit: 34000
Uncompressed size: 118017 bytes

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.