Fix missing default Accept header when options is an empty object

[soyuka]

Summary

Alternative fix for #177.

setHeaders() short-circuited on falsy options.headers, returning a plain {} instead of building Headers with the Accept default. This breaks @api-platform/admin's schemaAnalyzer.resolveSchemaParameters(), which always calls getParameters() with an explicit {}.

#177's fix (options.headers = {} instead of return) works, but mutates the caller-supplied options object as a side effect before falling through. This PR removes the early-return branch entirely and derives headers without mutating the input:

let headers =
  typeof options.headers === "function"
    ? options.headers()
    : (options.headers ?? {});

• No mutation of the caller's options object.
• Added a regression test asserting the default Accept header is sent when options is {}.
• Added a test asserting fetchJsonLd does not mutate the passed options object.

Test plan

• pnpm typecheck
• pnpm format:check
• pnpm test src/hydra/fetchJsonLd.test.ts (blocked locally by unrelated stray Yarn PnP files in this checkout; relying on CI)

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	71.18% (🎯 70%)	294 / 413
🔵	Statements	71.15% (🎯 70%)	296 / 416
🔵	Functions	70.31% (🎯 70%)	45 / 64
🔵	Branches	60.1% (🎯 58%)	238 / 396

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
src/hydra/fetchJsonLd.ts	100%	91.66%	100%	100%

Generated in workflow #319 for commit 468a076 by the Vitest Coverage Report Action