Go SDK: Align mode with SEP

[monikaicampbell]

Motivation and Context

The SEP-2624 spec defines interceptor modes as "active" | "audit". This change aligns the Go SDK's interceptor Mode value with the SEP spec.

How Has This Been Tested?

• go test ./... from go/sdk — all packages pass (chain, extension, integrations/gomiddleware).
• gofmt -l . — clean.
• go vet ./... — clean.

Breaking Changes

Yes:

• The exported constant interceptors.ModeEnforce is removed and replaced by interceptors.ModeActive. Callers referencing ModeEnforce must rename.
• The serialized mode value changes from "enforce" to "active". Any config expecting "enforce" must be updated.

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