🔍 fix: exclude deferred tools from instruction token accounting

[danny-avila]

Summary

• Fixes an over-count in AgentContext where tool definitions with defer_loading: true were included in toolSchemaTokens and getTokenBudgetBreakdown().toolCount even though they are never bound to the model until discovered via tool_search.
• This mismatch with the defer_loading filter in getEventDrivenToolsForBinding inflated the reported instruction token count and produced spurious context-overflow errors (e.g. 292 tools in a LibreChat MCP registry).
• Both counting paths now go through a new private getActiveToolDefinitions() helper that filters out defer_loading === true entries unless they are already present in discoveredToolNames.

Refs: LibreChat-AI/LibreChat#12702

Upgrade note for consumers

If a consumer caches toolSchemaTokens across runs and passes it back via AgentInputs.toolSchemaTokens (the fast-path added in 8e0ff93), calculateInstructionTokens is skipped entirely in fromConfig. A cached value computed before this fix will continue to reflect the inflated count. Consumers using that cache must invalidate it when upgrading to pick up the corrected accounting.

Out of scope (follow-ups worth filing)

• getActiveToolDefinitions() does not filter on allowed_callers — a code_execution-only tool passed via toolDefinitions still contributes to toolSchemaTokens even though it is never bound to the model. Pre-existing; narrower fix here keeps the change focused on the issue.
• toolSchemaTokens is a snapshot taken during calculateInstructionTokens and is not recomputed on markToolsAsDiscovered; toolCount is live. The JSDoc on getTokenBudgetBreakdown now documents this, and a test pins the snapshot semantic.

Test plan

• npx jest src/agents/__tests__/AgentContext.test.ts — 54/54 pass, including 5 new cases:
  • toolSchemaTokens excludes deferred-undiscovered entries
  • toolSchemaTokens includes deferred entries pre-seeded via discoveredTools
  • getTokenBudgetBreakdown().toolCount excludes deferred-undiscovered entries
  • getTokenBudgetBreakdown().toolCount reflects newly discovered deferred tools
  • toolSchemaTokens snapshot does not auto-update after markToolsAsDiscovered
• npx tsc --noEmit — clean
• npx eslint src/agents/AgentContext.ts src/agents/__tests__/AgentContext.test.ts — clean
• Full suite: only pre-existing live-API failures (missing ANTHROPIC_API_KEY / OPENAI_API_KEY), unrelated to this change