Review error codes (-32002 deprecation, renumbered protocol codes)

Review custom error codes (-32002 deprecation, renumbered protocol codes)

Part of #481.

DotNetMcp/Errors/McpErrorCodes.cs defines custom codes, including ResourceNotFound = -32002 and CapabilityNotAvailable = -32001.

SDK 2.0 / spec changes:

• SEP-2164: -32002 resource-not-found is deprecated in favor of -32602 InvalidParams under 2026-07-28 (SDK auto-selects by negotiated version).
• Protocol error codes renumbered: HeaderMismatch -32001 → -32020, MissingRequiredClientCapability -32003 → -32021, UnsupportedProtocolVersion -32004 → -32022. This frees -32001, but the custom CapabilityNotAvailable = -32001 should be re-confirmed for collisions.

Tasks

• Decide how to map ResourceNotFound: keep -32002 for legacy, prefer -32602 for 2026-07-28, or defer to the SDK's automatic selection.
• Confirm -32001/-32002 no longer collide with renumbered protocol codes.
• Update ErrorCodeDictionary/ErrorResultFactory and related docs as needed.

Acceptance

• Error-code mapping reviewed and aligned with the negotiated protocol version; no collisions.

Start by reading DotNetMcp/Errors/McpErrorCodes.cs, then trace ErrorCodeDictionary and ErrorResultFactory to understand how negotiated protocol versions select error codes. Compare the existing mappings with the cited SEP-2164 and SDK 2.0 changes; done means the version-aware mapping is reviewed, documented as needed, and has no code collisions.