* Add HTTP headers to Request

- Pass headers from request via ctx
- Add headers from ctx to CallToolRequest to further pass it to handlers.

* Add http-headers field to all Request Structs

* Use request_handler.tmpl to add http headers

* Add tests and documentation for Header passthrough

* Chore: fix comment in CallToolRequest struct

* Fix Race condition while setting context

Add early context cancellation check to prevent race condition where
SendRequest could return nil instead of context.Canceled when called
with an already-canceled context. This issue only manifested in certain
environments like GitHub Actions due to timing differences.

The fix ensures consistent behavior by checking context cancellation
before any I/O operations.

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Co-authored-by: opencode <noreply@opencode.ai>

* Embed client capabilities into the Session

Client sends its capabilities during the initialization step.

This commit embeds the client capabilities into the client session  in this step to enable
subsequent executions are able to check the client capabilities to determine
what actions they can perform. For instance, MCP Server checks the support of elicitation.
If elicititation is supported by the client MCP Server can send elicitation request.

* fix

* Add document for client capability based filtering

* Add return statement into the new example

* enhc(server): allow adding multiple resource templates at once

Following the same pattern from tools, prompts and resources, this patch allows
adding multiple resource templates at once.

* Apply coderabbitai suggestion

#483 (comment)

* Increase test coverage

Add a test case to cover the new function.

#483 (comment)

* feat: implement protocol version negotiation

Implement protocol version negotiation following the TypeScript SDK approach:
- Update LATEST_PROTOCOL_VERSION to 2025-06-18
- Add client-side validation of server protocol version
- Return UnsupportedProtocolVersionError for incompatible versions
- Add Mcp-Protocol-Version header support for HTTP transports
- Implement SetProtocolVersion method on HTTP connections
- Add comprehensive tests for protocol negotiation

This ensures both client and server agree on a mutually supported protocol
version, preventing compatibility issues.

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Co-Authored-By: opencode <noreply@opencode.ai>

* fmt

* refactor: improve protocol negotiation implementation

- Move HTTP header constants to common location to avoid duplication
- Add errors.Is interface to UnsupportedProtocolVersionError for better Go error handling
- Add comprehensive edge case tests for empty and malformed protocol versions
- Ensure consistent header constant usage across client and server packages

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Co-Authored-By: opencode <noreply@opencode.ai>

* fmt

* fix: include protocol version header in DELETE request for session termination

As per MCP specification, the MCP-Protocol-Version header must be included
on all subsequent requests to the MCP server, including DELETE requests
for terminating sessions.

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Co-Authored-By: opencode <noreply@opencode.ai>

* fix: maintain backward compatibility for protocol version negotiation

When the server does not receive an MCP-Protocol-Version header,
it should assume protocol version 2025-03-26 for backward compatibility
as per the MCP specification.

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Co-Authored-By: opencode <noreply@opencode.ai>

* test: update tests to reflect backward compatibility behavior

Tests now expect protocol version 2025-03-26 when no protocol version
is provided during initialization, as per MCP specification for
backward compatibility.

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Co-authored-by: opencode <noreply@opencode.ai>