feat: add ModifyRequest hook to client transports

[n33levo]

Summary

• add optional ModifyRequest callback to StreamableClientTransport and SSEClientTransport so callers can inject auth headers
• cover new hooks with transport tests to ensure requests carry custom headers across POST/GET/DELETE

Testing

• go test ./...

Fixes #533

[manuelibar]

@n33levo I think this PR duplicates this one

[jba]

@n33levo does this add anything new to? #546? If so, what? If not, please close.

[n33levo]

I implemented the ModifyRequest feature in PR #569, but I see @manuelibar has a more comprehensive implementation in PR #546 that covers the same functionality. I'm closing my PR to avoid duplication. The feature works great and addresses the original request perfectly!