fix: serialize IncludeContext with MCP values

[juliuss007]

Summary

• serialize IncludeContext::ThisServer and IncludeContext::AllServers as MCP-compatible camelCase values
• keep IncludeContext::None as none
• add regression coverage for serialization and deserialization of the MCP values

Fixes PlatformNetwork/bounty-challenge#52972

Verification

• cargo +1.91.1 test -p cortex-mcp-types include_context_uses_mcp_camel_case_values
• cargo +1.91.1 check -p cortex-mcp-types
• git diff --check HEAD^ HEAD
• cargo +1.91.1 fmt --package cortex-mcp-types --check

Bounty claim: PlatformNetwork/bounty-challenge#52972