Fix stream consumption in MCP adapter proxy (#97)

## Summary

Fixes #48.

The adapter proxy directly wrapped non-seekable incoming request bodies
in `StreamContent`. When the request content was consumed more than
once,
this resulted in:

`System.InvalidOperationException: The stream was already consumed. It
cannot be read again.`

This change enables buffering for non-seekable request bodies before
creating
the proxied `StreamContent`.

## Tests

- Added regression coverage for non-seekable request bodies.
- `dotnet test`
- 251 tests passed
- 0 failed

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Co-authored-by: Rakshit Singh <rakshitsinghhh@gmail.com>