feat: trace Letta Agent SDK sessions

[KarthikS (karthiksenv)]

Fixes #2577.

Adds automatic instrumentation for @letta-ai/letta-agent-sdk >=0.8.28 <0.9.0 through the standard transformer, channels, and plugin. Both the main and client entrypoints are covered. Agent creation and session resumption produce spans; a turn stays open from send() through the terminal stream() result, with tool child spans and error/cancellation handling. sendAndWaitForResult() is captured through the same path.

Also exports wrapLettaAgentSDK for manual module/client/session wrapping, preserves SDK return values and stream identity, prevents duplicate spans when combined with automatic instrumentation, and supports the integration opt-out. Client credentials and inline base64 image bytes are excluded from the logged configuration/input.

Validation on Node 24.18.0 with pnpm 10.33.0:

• Reproduced missing spans using the published SDK before registration: 12 failing contract tests. With the fix, the targeted plugin/registry/provenance suite passes all 174 tests.
• All 23 Letta contract tests pass against pinned 0.8.28 and separately against published 0.8.39, including both package entrypoints, queued/concurrent turns, errors, cancellation, and manual/automatic deduplication.
• Built-package Node loader-hook smoke checks pass: two expected spans when enabled, zero with BRAINTRUST_DISABLE_INSTRUMENTATION=letta.
• SDK build, production/test TypeScript checks, frozen-lockfile validation, formatting, changeset validation, and git diff --check pass. ESLint reports zero errors and 13 type-assertion warnings.

This is a draft pending real-provider e2e coverage. LETTA_API_KEY was unavailable, so provider cassette recording and replay remain unverified. The contract tests run the actual published SDK with stubbed runtime transport; they are not provider e2e tests, and no provider recordings were fabricated.