docs(js): fix httpHeaders dataCollection examples

[vianmangal]

DESCRIBE YOUR PR

Fixes #18685.

Updates JavaScript dataCollection.httpHeaders examples to use the supported nested request and response shape. The correction covers the configuration reference, data-management guide, HttpClient integration guide, and SDK foundation examples so nearby examples do not keep suggesting the invalid flat shape.

IS YOUR CHANGE URGENT?

• Urgent deadline (GA date, etc.): YYYY-MM-DD
• Other deadline: YYYY-MM-DD
• No deadline: Not urgent, can wait up to 1 week+

VERIFICATION

• Compiled all four changed files with @mdx-js/mdx
• Confirmed no flat httpHeaders: true, httpHeaders: false, httpHeaders: { allow: ... }, or httpHeaders: { deny: ... } examples remain in the JavaScript and SDK foundation docs
• git diff --check passes

SLA

• Teamwork makes the dream work, so please add a reviewer to your PRs.
• Please give the docs team up to 1 week to review your PR unless you've supplied a deadline.

Thanks in advance for your help!

PRE-MERGE CHECKLIST

• Checked Vercel preview for correctness, including links
• PR was reviewed and approved by any necessary SMEs (subject matter experts)
• PR was reviewed and approved by a member of the Sentry docs team

LEGAL BOILERPLATE

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

AI ASSISTANCE

I used Codex to identify related invalid examples and run repository checks. I reviewed the resulting diff and verification output.

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