feat(conformance): add SEP-3221 Secure Parameters traceability mapping

[anubhav756]

This PR adds the structured traceability mapping file (src/seps/sep-3221.yaml) for the Secure Parameters Extension (io.modelcontextprotocol/secure-params).

Motivation and Context

Under SEP-2484 (Conformance Tests Required for Final SEPs), all extension proposals require a complete YAML traceability mapping connecting every RFC 2119 normative assertion (MUST, MUST NOT, SHOULD) to discrete conformance test check IDs.

This PR establishes the formal traceability contract for the Secure Parameters Extension before automated test scenarios and harness checks are authored.

How Has This Been Tested?

• Validated YAML syntax, structure, and check naming conventions against existing traceability files (src/seps/sep-2663.yaml and src/seps/sep-2575.yaml).
• Audited all 13 normative assertions to ensure wire-level observability and determinism.
• Verified that host-internal requirements (e.g. prompt rendering isolation) are formally declared under excluded in compliance with SEP-2484.

Breaking Changes

None. This adds a new extension traceability mapping file and does not modify core test suites.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

• Associated Specification PR: feat(sep): add SEP-3221 Secure Parameters Extension modelcontextprotocol#3221 (SEP-3221)

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commit: 5b87412