[Agent Builder] Improve dynamic attachments: optional tool params, skill discovery, and tracer utilities

[patrykkopycinski]

Summary

• Make selectTools parameters optional (skills, filestore, experimentalFeatures) with sensible defaults, enabling callers that don't need skill/filestore integration (e.g., deep agent mode) to use the function without providing all dependencies
• Add mandatory skill discovery step to the research agent prompt when the skills experimental feature is enabled, ensuring skills are loaded before general-purpose tools
• Strengthen skill loading instructions in skill prompts to enforce protocol compliance (loading skills via filestore.read before calling other tools)
• Add createTracer, createTracerFactory, and getAgentBuilderTracer utilities for OpenTelemetry tracing in the agent_builder plugin
• Add barrel export for kbn-langchain server tracers (APMTracer, TelemetryTracer, LangSmith helpers)
• Update kbn-langchain tracers README with structured documentation covering all available tracers

Test plan

• Verify agent builder conversations work correctly with default agent mode
• Verify skill discovery works when experimentalFeatures.skills is enabled
• Verify tracing utilities create valid OpenTelemetry tracers
• Verify the selectTools function works when optional parameters are not provided

Production-Readiness Checklist — Agent Skills Ecosystem

Generated against [Epic] Creation of the Agent Skills Ecosystem for Elastic Security.

Narrative role: Observability pillar. Exposes OTEL tracers so we can measure invocation frequency and accept/reject rates — the vision's "if we cannot measure it, we cannot responsibly scale it" requirement.

Must-do before this can ship

• Make createTracer / getAgentBuilderTracer the only supported path — add an ESLint rule forbidding direct APMTracer / TelemetryTracer imports in Agent Builder plugins
• Deterministic eval: fails if the research agent calls any tool before a skill-load in the presence of the skills experimental flag (enforces the "skill discovery first" contract)
• Propagate trace_id into SSE events so the UI "why did it do that" surface can deep-link into the trace
• Bundle-size budget check for the new kbn-langchain server barrel — the re-exports land on every AB server bundle

Follow-ups (post-merge)

• Document the tracer utility as the canonical pattern in @kbn/evals README (pair with docs(evals): vision alignment — trace-first guidance, JSDoc, and public API documentation #258224)
• Emit standard span attributes (skill id, tool id, connector id, user id) so the Monitoring tab from feat(evals): AESOP spike + eval platform for Agent Builder skills #261057/feat(evals): add Suites tab for evaluation suite management #261059 can aggregate without per-feature wiring

[elasticmachine]

🤖 Jobs for this PR can be triggered through checkboxes. 🚧

ℹ️ To trigger the CI, please tick the checkbox below 👇

• Click to trigger kibana-pull-request for this PR!
• Click to trigger kibana-deploy-project-from-pr for this PR!
• Click to trigger kibana-deploy-cloud-from-pr for this PR!
• Click to trigger kibana-entity-store-performance-from-pr for this PR!
• Click to trigger kibana-storybooks-from-pr for this PR!