perf(request): reduce overhead by lazy-loading headers for json/form data

[0x676e67]

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[gemini-code-assist]

Summary of Changes

Hello @0x676e67, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a performance enhancement by refactoring how certain HTTP headers are set within the RequestBuilder. By switching from or_insert to or_insert_with, the creation of HeaderValue instances is now lazy-loaded. This means the header value is only computed and allocated if the header is not already present, thereby reducing overhead and improving efficiency, particularly for requests involving multipart, form, or JSON bodies where these headers might be frequently set.

Highlights

• Performance Optimization: Replaced or_insert with or_insert_with for header insertions, deferring the creation of HeaderValue until it's actually needed. This avoids unnecessary allocations and computations when a header already exists.
• Targeted Headers: The optimization applies to CONTENT_LENGTH for multipart form data and CONTENT_TYPE for application/x-www-form-urlencoded and application/json bodies.

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[gemini-code-assist]

Code Review

This pull request improves performance by using or_insert_with to lazily initialize headers for multipart, form, and JSON data. This is a good optimization, especially for multipart forms where calculating the content length can be non-trivial. For form and json data, the use of HeaderValue::from_static is already very efficient as it's a const fn, so the performance gain is likely negligible, but the change is good for consistency. I've added one minor suggestion to improve the formatting and make the code more idiomatic in the form method.