docs: add Xquik API-key example and redact header logs

[kriptoburak]

What changed

• Added a Streamable HTTP Xquik example using the correct x-api-key header.
• Keeps the raw key in an environment variable and forces HTTP-only transport.
• Fixed existing custom-header logging so secret header values are never printed or written to debug logs.
• Added a regression test proving header values remain absent from logs.

Validation

• 104 unit tests passed.
• Prettier and TypeScript checks passed.
• Production build passed.
• Diff checks passed.

The header log redaction is independent of the Xquik example and protects every custom-header user.

[chatgpt-codex-connector]

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