Go SDK: Align mode with SEP (#40)

<!-- Provide a brief summary of your changes -->

## 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
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [x] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [x] I have added or updated documentation as needed

## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->

Signed-off-by: monikaicampbell <mcampbell350@bloomberg.net>

go/sdk/doc/DESIGN.md

@@ -174,7 +174,7 @@ Response-phase validators see the post-mutation payload.
 
 ### Validator execution
 - All matching validators run in parallel (goroutines).
-- A validator returning `Valid: false` with `Severity: "error"` in enforced
+- A validator returning `Valid: false` with `Severity: "error"` in active
   mode aborts the chain.
 - `FailOpen: true` validators log errors and record an `InvokeResult`
   for observability, but don't abort.