docs(telemetry): add search_sentry_tools recipe and result count pivots

[dcramer]

Add telemetry documentation for the search_sentry_tools catalog discovery pattern.

What's added:

• Three new Investigation Pivots: mcp.tool.name, gen_ai.tool.call.result, gen_ai.tool.call.result.count
• Query recipe for zero-result search_sentry_tools catalog calls (covers both the old search_tools name and the current search_sentry_tools name via OR)
• Updated MCP Tool Execution domain to mention catalog discovery and the result count pattern
• Attribute notes for all three new fields, including cardinality warnings on gen_ai.tool.call.result

Why: PR #1072 added gen_ai.tool.call.result and gen_ai.tool.call.result.count to search_sentry_tools spans but TELEMETRY.md wasn't updated. The mcp.tool.name field was also absent. Without this, zero-result queries are invisible in telemetry and there's no documented recipe for investigating tool catalog gaps.

Query syntax verified against production spans dataset.

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