fix: resolve global state pollution causing random-order test failures in HTTP suite

[gr8man]

Description
This PR resolves multiple test pollution issues causing execution failures when tests in the HTTP system suite are executed in random order (--order-by=random).
Ref: #9968
Key Changes:

• system/HTTP/IncomingRequest.php: Set the globally retrieved Negotiator service to non-shared (Services::negotiator($this, false)) to prevent cross-request negotiation state bleeding.
• tests/system/HTTP/UserAgentTest.php, tests/system/HTTP/NegotiateTest.php: Added class-level #[BackupGlobals(true)] and called $this->resetServices() in setUp() to restore mutated superglobals and clear mock services.
• tests/system/HTTP/RedirectResponseTest.php, tests/system/HTTP/ResponseTest.php, tests/system/HTTP/DownloadResponseTest.php: Added #[BackupGlobals(true)] to prevent superglobals mutation leakage.
• tests/system/HTTP/ResponseTest.php: Injected a clean, empty superglobals mock inside testSetLink to isolate the URL generation from dirty query string pollution (e.g. code=good).
• tests/system/HTTP/IncomingRequestTest.php: Corrected unmatched close parenthesis typos in header strings and corrected testNegotiatesLanguage assertion (changed expected output from 'en' to 'da') that was previously green only because of stale negotiator cache pollution.
  Checklist:
• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[gr8man]

IMPORTANT

Note to reviewers and developers: Please ensure that tests are run regularly with the --order-by=random flag during development. Identifying test pollution and global state leakage bugs at a later stage is extremely difficult and time-consuming, as these failures only manifest under very specific test suite execution orders.