fix(training-agent): remove signal_id/destination aliases from activate_signal, clarify schema descriptions

[bokelley]

Refs #3349 — the adcp-client scenario fix (scenarios/signals.js) must be addressed in that repo separately.

Summary

• Removes signal_id (wrong field), destination (singular), and options SDK-compat aliases from the training agent's activate_signal tool definition; these shims were silently accepting the exact malformed payload reported in Storyboard 5.13 activate_signal scenario uses wrong field name and wrong type for signal identifier #3349 instead of surfacing it as an error.
• Adds missing idempotency_key to the tool inputSchema properties and updates required to ['signal_agent_segment_id', 'destinations', 'idempotency_key'], matching both the wire schema (activate-signal-request.json) and the runtime dispatcher enforcement.
• Sharpens signal_agent_segment_id description in activate-signal-request.json and get-signals-response.json to make the distinction from the signal_id catalog object explicit at the schema level.

All three compliance storyboard YAMLs (signals_baseline, signal-owned, signal-marketplace) already use the correct fields — this aligns the training agent implementation with those storyboards.

Non-breaking justification: removes non-spec aliases from a training agent tool definition; no wire-format or schema type changes. The compliance storyboard source YAMLs and the published activate-signal-request.json required fields are unchanged. Callers sending correct protocol fields are unaffected.

Routing note: this is a patch changeset. No 3.0.x branch exists — targeting main. Needs @bokelley to cut a patch branch or merge into the upcoming 3.0.1 milestone once the branch is open.

Pre-PR review

• code-reviewer: approved — two blockers fixed (missing idempotency_key in schema, empty required array); no remaining blockers
• ad-tech-protocol-expert: approved — non-breaking per spec; signal_id/signal_agent_segment_id distinction is a deliberate two-concept design (catalog ref vs. opaque activation handle); storyboard YAMLs already correct

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
  fix → push.
• Or re-trigger: comment /triage execute on the source
  issue.

See #3121
for context.

Session: https://claude.ai/code/session_011S9bnPFjuLTCyPPEUD2AoD

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