fix: apply requestBuilder headers when sending rpc messages

[dead8309]

fixes: #63

Adds the missing requestBuilder call to send function

Motivation and Context

Client doesn't send Auth headers when sending a rpc message

How Has This Been Tested?

logs:

2025-05-05 01:07:32.569 12997-13028            V  REQUEST: https://[REDACTED]/sse/message?sessionId=c28e21031144b7b85616b23d6caf8c7feb1f88e88ad1beefb533b68060d3e236
                                                  METHOD: POST
                                                  COMMON HEADERS
                                                  -> Accept: application/json
                                                  -> Accept-Charset: UTF-8
                                                  -> Authorization: Bearer e41ef9271332d4732fb4f[REDACTED]
                                                  CONTENT HEADERS
                                                  -> Content-Length: 230
                                                  -> Content-Type: application/json
2025-05-05 01:07:32.594 12997-13028            D  tagSocket(114) with statsTag=0xffffffff, statsUid=-1
2025-05-05 01:07:33.097 12997-13055            V  RESPONSE: 202 Accepted
                                                  METHOD: POST
                                                  FROM: https://[REDACTED]/sse/message?sessionId=c28e21031144b7b85616b23d6caf8c7feb1f88e88ad1beefb533b68060d3e236
                                                  COMMON HEADERS
                                                  -> Access-Control-Allow-Origin: *
                                                  -> CF-RAY: 93aa74118ec0938b-BBI
                                                  -> Cache-Control: no-cache
                                                  -> Connection: keep-alive
                                                  -> Content-Length: 8
                                                  -> Content-Type: text/event-stream
                                                  -> Date: Sun, 04 May 2025 19:37:34 GMT

Breaking Changes

No

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[e5l]

Hey @dead8309, thank you for the contribution! LGTM