[Schema] SEP-2164: Switch resource-not-found error code to -32602 with protocol-version-aware fallback

[chr-hertel]

Implements SEP-2164 for the MCP Spec 2026-07-28 release.

Tracked by umbrella #337.

Spec summary

Resource-not-found now uses JSON-RPC -32602 (Invalid Params) instead of the MCP-custom -32002, aligning with JSON-RPC.

PHP SDK changes

• src/Schema/JsonRpc/Error.php:37 defines RESOURCE_NOT_FOUND = -32002. Change to -32602, or deprecate the constant and add RESOURCE_NOT_FOUND_INVALID_PARAMS = -32602.
• Error::forResourceNotFound() (line ~109) is called from ReadResourceHandler, ResourceSubscribeHandler, ResourceUnsubscribeHandler, GetPromptHandler, CompletionCompleteHandler — all flip automatically once the constant changes.
• Negotiate by protocol version: pre-2026-07-28 clients still expect -32002. Add version-aware dispatch in Error::forResourceNotFound() reading session/request protocol version.
• Update conformance & unit tests asserting the old code.

Related

• Umbrella [2026-07-28] Schema modernization & error code standardization #337