Add RegisterTools API to McpClient for pre-populating tool cache

[tarekgh]

Summary

This PR adds a RegisterTools API to McpClient that allows pre-populating the internal tool cache with tool definitions. This enables MCP clients to send \Mcp-Param-*\ HTTP headers (based on \x-mcp-header\ schema annotations) without requiring a prior \ListToolsAsync\ call.

Fixes #1577

Changes

API Addition

• *\McpClient.RegisterTools(IEnumerable)* — registers tool definitions in the client's tool cache, enabling the transport to send \Mcp-Param-*\ headers for those tools

Implementation Details

• Registered tools are added to the same _toolCache\ used by \ListToolsAsync\
• A thread-safe \ConcurrentDictionary<string, byte>\ tracks registered tool names (used as a concurrent set)
• \ToolCacheClearing\ (invoked by \ListToolsAsync) only removes server-discovered tools; registered tools survive cache clears
• Fast path: when no tools are registered, _toolCache.Clear()\ is called directly
• Tools with invalid \x-mcp-header\ annotations are validated and rejected with logging
• Re-registering a tool with the same name overwrites the previous definition (last write wins)
• If the server returns a tool with the same name as a registered tool, the server's definition overwrites in the cache, but the tool retains its registered/pinned status

Cache Interaction Behavior

• Register → ListToolsAsync: registered tools survive the clear, server tools added alongside
• ListToolsAsync → Register: both coexist in cache
• Register → ListToolsAsync → Register: all registered tools survive, server tools refreshed

Tests

Unit Tests (11 tests)

• Register then list — server tools repopulated correctly
• Multiple ListToolsAsync cycles with registered tools
• List then register ordering
• Register → list → register again
• Same name as server tool (server overwrites definition, pinned status preserved)
• Invalid x-mcp-header schema rejection
• Duplicate registration (last write wins)
• Null argument validation
• No-header-annotation tools still accepted
• Register then CallToolAsync without ListToolsAsync (cache lookup works)

HTTP Integration Tests (3 tests)

• Core scenario: \RegisterTools\ → \CallToolAsync\ (NO \ListToolsAsync) → verify \Mcp-Param-Region\ and \Mcp-Param-Priority\ headers received by server
• No headers sent without register or list
• Registered tool headers survive \ListToolsAsync\ cache clear

[tarekgh]

#1553