Send spec-compliant Accept header in fetchers

[trevor-scheer]

Summary

• Send application/graphql-response+json as the preferred content type per the GraphQL over HTTP spec, falling back to application/json;q=0.9
• Applied to both createSimpleFetcher (which previously sent no Accept header) and createMultipartFetcher (which sent application/json, multipart/mixed)

Closes #4189

Changes

• createSimpleFetcher: added accept: 'application/graphql-response+json, application/json;q=0.9'
• createMultipartFetcher: changed accept from application/json, multipart/mixed to application/graphql-response+json, application/json;q=0.9, multipart/mixed
• Both fetchers continue to allow user-provided headers (options.headers, fetcherOpts.headers) to override the default
• Added acceptHeaders.spec.ts with tests covering default headers and override behavior

Validation Steps

1. yarn dev:graphiql to start the dev server
2. Open GraphiQL in the browser and run a query (e.g. { __typename })
3. In DevTools Network tab, inspect the fetch request to the GraphQL endpoint — the Accept request header should be application/graphql-response+json, application/json;q=0.9, multipart/mixed

[changeset-bot]

🦋 Changeset detected

Latest commit: bceaa66

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@graphiql/toolkit	Minor
@graphiql/plugin-history	Patch
@graphiql/react	Patch

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.96%. Comparing base (7979bf5) to head (bceaa66).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   63.96%   63.96%           
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  Files          35       35           
  Lines        3086     3086           
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  Hits         1974     1974           
  Misses       1107     1107           
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see 1 file with indirect coverage changes

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[TallTed]

The currently specified Accept: header —

application/graphql-response+json, application/json;q=0.9, multipart/mixed

— persists an error. Leaving off the quality rating causes a media type to be treates as if 1.0 (maximum) quality, and that's what the old Accept: header had, and what the new header persists.

I'm fairly well certain that one of application/graphql-response+json or multipart/mixed is really preferred over the other, but I'm not sure which that is.

For sake of understanding, I offer two options. The first prefers application/graphql-response+json and the second prefers multipart/mixed.

application/graphql-response+json, application/json;q=0.9, multipart/mixed;q=0.95

application/graphql-response+json;q=0.95, application/json;q=0.9, multipart/mixed

Note — both of my offered options presume that application/json is the least-preferred. I hope that what I've written is clear enough that any reader of this comment can figure out how to make application/json more preferred than either of the other two.

[dimaMachina]

Suggested change

	const QUERY = '{ __typename }';
	const BASE_URL = 'http://localhost';

I don't want to be annoying, but these vars are repeated 5 times each 😄