Add SEP-1303 to changelog

Add changelog entry for input validation errors as tool execution errors.

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Co-Authored-By: Claude <[email protected]>

Author: dsp-ant

docs/specification/draft/changelog.mdx

@@ -18,6 +18,7 @@ the previous revision, [2025-06-18](/specification/2025-06-18).
 
 1. Clarify that servers must respond with HTTP 403 Forbidden for invalid Origin headers in Streamable HTTP transport. (PR [#1439](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1439))
 2. Updated the [Security Best Practices guidance](https://modelcontextprotocol.io/specification/draft/basic/security_best_practices).
+3. Clarify that input validation errors should be returned as Tool Execution Errors rather than Protocol Errors to enable model self-correction ([SEP-1303](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1303)).
 
 ## Other schema changes