fix(training-agent): create fresh MCP Server per request to eliminate transport-binding race

[bokelley]

Closes #4084

The tenant router reused a singleton MCP Server per tenant (held by TenantRegistry) and called server.connect(transport) on every HTTP POST. Under concurrent load the MCP SDK rejected re-connection with "Already connected to a transport", producing -32603 Internal server error on ~1/3 of cold-start requests. The compliance heartbeat and storyboard runners targeting the training agent both hit this under back-to-back load.

Fix: Add createServer(tenantId) factory to RegistryHolder in registry.ts. The factory stores a closure per tenant (keyed by cfg.tenantId — same key TenantRegistry uses) that calls createAdcpServerFromPlatform(opts) fresh per invocation. Each HTTP POST in tenantMcpHandler now calls holder.createServer(resolved.tenantId) to get its own Protocol instance, then runs the connect → handle → close lifecycle on that instance. The registry singleton is retained only for resolveByRequest (tenant existence and health checks).

This matches the pattern already used in standalone.ts and the legacy/strict MCP handlers in index.ts, and matches the MCP SDK's stated requirement for stateless mode (sessionIdGenerator: undefined): one Server per connection.

Non-breaking justification: purely internal server lifecycle change — no change to the HTTP interface, JSON-RPC wire format, tenant registration, or any public API surface. Existing clients are unaffected.

Pre-PR review:

• code-reviewer: approved — closure correctness verified, factory-key fix applied (cfg.tenantId not id), ordering guarantee tight
• dx-expert: approved — race correctly fixed, resolved.server no longer used for transport ops, comment adequate

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See #3121
for context.

Session: https://claude.ai/code/session_011KzNaC6rJnpxvUe4QCMUkd

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