fix(proxy): handle forwarded request headers beyond stack buffer

[binarylogic]

Summary

Make request header forwarding resilient when header count exceeds the fixed stack buffer.

Why

Forwarding previously used a fixed [64]std.http.Header buffer and could fail with TooManyHeaders. This caused avoidable request failures for high-header inputs.

Changes

• Count forwardable headers first.
• Use stack buffer when count fits.
• Allocate a dynamic header slice in request arena when count exceeds stack capacity.

Validation

• zig build test

[jaronoff97]

we don't do this right now so we can avoid any extra allocation in this path, do you think we'll be seeing a lot of cases with more than 64 headers?

[github-actions]

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