agentHost: adopt OTEL log channel, support logs from remote agent hosts

[connor4312]

Wires up the new channels-otlp/ protocol so the agent host's ILogService is mirrored over an ahp-otlp://logs/{level} channel and surfaced as a per-host Output channel in the workbench.

What changed

Agent host (server)

• New OtlpLogEmitter / OtlpEmitterLogger in platform/agentHost/common/otlp/. LogService is constructed with the OTLP logger as a secondary sink in both agentHostMain.ts and agentHostServerMain.ts so every log call fans out to subscribers.
• ProtocolServerHandler advertises telemetry.logs in InitializeResult, routes subscribe/unsubscribe on ahp-otlp: channels through a typed ChannelSubscription discriminated union, canonicalises the channel URI per-level, and broadcasts otlp/exportLogs notifications filtered per subscriber severity.

Workbench (client)

• RemoteAgentHostProtocolClient captures the full InitializeResult, exposes subscribeStateless, and fires onDidReceiveOtlpLogs for inbound batches.
• New RemoteAgentHostLogForwarder (in sessions/contrib/providers/remoteAgentHost/browser/) registers a per-host Agent Host (${host}) Output channel via IOutputChannelRegistry, subscribes at the workbench's ILogService level (re-subscribes on change), and appends decoded records. Constructed from remoteAgentHost.contribution.ts::_setupConnection so it covers the WebSocket, SSH and tunnel paths uniformly. Local agent host IPC logging is unchanged.
• Existing remote IPC traffic channel renamed to Agent Host IPC (${host}) to disambiguate from the new OTLP-derived channel.

Shared utilities

• UriTemplate moves from workbench/contrib/mcp/common/ to base/common/ so the forwarder can use UriTemplate.parse(template).resolve({ level }) for spec-conformant {level} expansion. MCP imports updated.

Why

The remote agent host already produces structured logs via its ILogService, but until now there was no path for clients to observe them. With this change a remote host's logs (whether spawned over WebSocket, SSH or a dev tunnel) automatically appear in the user's Output panel as Agent Host (${name}), alongside the existing Agent Host IPC (${name}) wire-traffic channel.

Notes for reviewers

• The local agent host still uses its IPC LoggerChannel (out-of-bounds log forwarding) — only remote connections opt into the OTLP path. This avoids duplicating the existing local Output channel.
• Output channels are intentionally never deregistered. The set of remote hosts is small and persisting the channel after disconnect lets the user inspect prior log output.
• _attach short-circuits while not Connected so reconnect cycles don't leak stale subscriptions if the workbench log level changes mid-drop.
• Server canonicalises the OTLP channel URI on subscribe so URI variants (query/fragment) that parse to the same level collapse to one entry per (client, level).

Tests

• 8 unit tests for OtlpLogEmitter and helpers (level mappings, payload encode/decode, template expansion).
• 7 unit tests for the protocol server's OTLP routing (advertise / subscribe / unsubscribe / multi-level / disconnect / unknown URI / URI variant collapse).
• 3 end-to-end integration tests against a real spawned server (otlpLogs.integrationTest.ts).
• 25 UriTemplate tests (moved alongside the file).
• 175 MCP tests still pass after the move; 810 wider AgentHost and 194 RemoteAgentHost tests green.
• Compile, layer check, and ESLint all clean.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com