fix: pass SEP-2243 client conformance

[DaleSeo]

Fixes #978

Motivation and Context

Updates the Rust conformance client to pass the SEP-2243 standard, custom, and invalid tool-header scenarios. Header scenarios now negotiate the draft protocol through discovery, consume the conformance runner's toolCalls context, and reject malformed x-mcp-header tools before they reach callers.

The client previously initialized before draft request metadata was available, ignored camelCase tool-call context, and left invalid header-annotated tools visible even after rejecting them from the header cache.

How Has This Been Tested?

The conformance runner passes all checks for http-standard-headers (3/3), http-custom-headers (18/18), and http-invalid-tool-headers (11/11).

Breaking Changes

None.

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