feat(observability): add LangSmith as alternative tracing backend

[ngoclam9415]

What

Adds LangSmith as an alternative tracing backend for @observable, mirroring the existing Langfuse integration. ONE backend active per process; selection at decoration time: LangSmith > Langfuse > no-op. No call-site changes — all 30+ @observable sites (agents, workflows, LLM calls, codecs) traced automatically.

Why

Operators want LangSmith as an option alongside Langfuse. Both are OTLP-capable, but a parallel backend behind the existing decorator is lower-risk than an OTLP pivot and matches the established LANGFUSE_ENABLED toggle pattern.

How to enable

pip install dana[observability]
export LANGSMITH_TRACING=true LANGSMITH_API_KEY=lsv2_...
# optional: LANGSMITH_PROJECT, LANGSMITH_ENDPOINT
# alias: DANA_LANGSMITH_ENABLED=true|1|yes

Toggle back to Langfuse by unsetting LANGSMITH_TRACING and setting LANGFUSE_ENABLED=true.

Changes

• dana/common/observable.py — dispatch (LANGSMITH>LANGFUSE>noop), _langsmith_kwargs adapter (as_type→run_type, session_id/user_id→metadata), bare-form normalization (avoids double-call), factored _langfuse_wrap preserving byte-equivalent flush behavior on the langfuse path.
• dana/__init__/init_environment.py — _install_langsmith_shim mirrors the langfuse shim (clean import without the extra).
• pyproject.toml — langsmith>=0.8,<0.9 in [observability] extra (langfuse pins untouched).
• tests/unit/test_observable_langsmith_dispatch.py — 11 mocked cases incl. SC7 langfuse flush regression (sync/bare/async) and async exclusivity.
• docs/codebase-summary.md — tracing-backends section + silent no-op caveat.
• docs/project-changelog.md — Added + Changed entries.

Verification

• 11/11 new tests pass; full unit suite 1345 pass, 20 skip, 1 xfail — no regressions.
• ruff + ruff-format clean; pre-commit hooks pass.
• Smoke-tested all 3 dispatch modes + shim path against real langsmith==0.8.5.
• Code-reviewed: langfuse path verified byte-equivalent (flush fires once for sync/bare/async); exclusivity confirmed live.

Scope / Out of scope

• Phase 1 (this PR): tracing dispatch only.
• Phase 2 (deferred): LangSmithPromptRepository (prompt management) — separate PR. LangSmith native versioning will also let us drop the dana_versions config-stashing hack currently in LangfusePromptRepository.
• Deferred follow-ups: symmetric runtime-warning when an env flag is set but its package is missing (currently silent no-op, parity with langfuse); per-call-site run_type="llm" for llm_caller.py spans; process_inputs scrubbing for self (parity with langfuse).

Breaking changes

None. Default behavior (no env set) is unchanged except @observable now returns the decorated function unchanged (identity) on the no-op path instead of a passthrough wrapper — improves inspect.signature(), no introspection-sensitive call sites affected.