feat: add Request.read(size) method for partial body reading

[Jagdish1115]

Summary

Adds a read(size) method to the Request class that reads exactly size bytes from the request body at a time. Currently there is no way to read a specific number of bytes from the request body. The only options are body() which loads everything into memory at once, or stream() which gives server-decided chunk sizes.
This is useful for processing large request bodies in chunks without loading everything into memory — for example, saving portions of a large body to multiple files.

Relates to #3113
Example usage:
chunk1 = await request.read(1024) # first 1KB
chunk2 = await request.read(1024) # next 1KB
chunk3 = await request.read(-1) # rest of body

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[Jagdish1115]

Hi,

This is my first open source contribution, so I appreciate any feedback or guidance.
I've implemented the read(size) method on the Request class as discussed in #3113. The method reads up to size bytes from the request body at a time using an internal buffer, avoiding loading the entire body into memory at once.
Regarding the coverage failure — I noticed the main branch is already at 89% coverage before my changes. I've added tests for the main scenarios but I'm still working on fully covering line 259. Any guidance would be greatly appreciated.