Add Python LangChain local OTLP mode

[RitwijParmar]

Summary

Part of #20.

This adds a scoped local-development path for the Python LangChain integration:

from contextcompany.langchain import instrument_langchain

instrument_langchain(local=True)

In local mode, the SDK no longer requires TCC_API_KEY. It configures the LangChain OpenTelemetry exporter against a local OTLP HTTP endpoint instead:

• default: http://localhost:4318/v1/traces
• override with tcc_url=...
• or set TCC_LOCAL_OTLP_ENDPOINT

This does not attempt to solve the full local UI/collector story in one PR, but it removes the current cloud-auth hard stop for Python LangChain/LangGraph local development and makes it possible to run against a local collector/proxy without patching env vars or passing a fake API key.

Changes

• Add local: bool = False to instrument_langchain(...)
• Allow create_otlp_exporter(...) to omit the Authorization header when no API key is provided
• Add TCC_LOCAL_OTLP_ENDPOINT / default local OTLP endpoint resolution
• Document Python LangChain local mode in packages/python/README.md
• Add regression tests for local mode, explicit local endpoint override, cloud-mode API key behavior, and auth-header omission

Validation

• /tmp/codex-observatory-venv/bin/python -m pytest packages/python/tests/test_langchain_local.py -> 4 passed
• python -m ruff check packages/python/contextcompany/config.py packages/python/contextcompany/langchain/base.py packages/python/contextcompany/langchain/__init__.py packages/python/tests/test_langchain_local.py
• python -m ruff format --check packages/python/contextcompany/config.py packages/python/contextcompany/langchain/base.py packages/python/contextcompany/langchain/__init__.py packages/python/tests/test_langchain_local.py
• git diff --check

[changeset-bot]

🦋 Changeset detected

Latest commit: b66884d

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 0 packages

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[RitwijParmar]

Added an explicit changeset note in b66884d for the Python LangChain local OTLP mode, following the changeset-bot guidance. Since this PR only touches the Python package/docs/tests and not a JS workspace package, the changeset keeps the version frontmatter empty while documenting the change.