MCP: close the command context when a tool call ends

[erichare]

What this PR does:

MCP tool calls build a CommandContext, which puts the request ID and schema names into the logging context (MDC), but nothing removes them when the call ends. The HTTP resources close the context on termination; McpResource does not. Keyspace tools also build a second RequestContext for the command context, so one call starts under one request ID and runs under another.

This PR makes the MCP path do what GeneralResource already does:

• McpResource.processCommand() closes the command context with onTermination(), so it runs on success, on failure, when the MCP feature is disabled, and on cancellation. A failure while closing is logged and does not replace the tool result or the original error.
• McpResource.buildKeyspaceContext() passes the RequestContext it already created to the context builder instead of creating a second one. Database and collection/table calls already did this.

Which issue(s) this PR fixes:
Part of #2629. Leaves #2629 open for the trace-context audit, see NOTE above.

Checklist

• Changes manually tested
• Automated Tests added/updated
• Documentation added/updated
• CLA Signed: DataStax CLA