[codex] fix Worker create_agent vendoring

[CollierKing]

Summary

Fixes the Worker example dependency layout after PR #44 was already merged.

• Removes the direct langchain dependency from examples/workers/pyproject.toml so pywrangler sync only resolves Pyodide-compatible dependencies.
• Keeps the known-working Worker create_agent stack in scripts/setup_pyodide_deps.sh: langchain==1.0.0, langchain-core==1.1.0, langgraph==1.0.0, langgraph-sdk==0.3.1, and langgraph-checkpoint==2.1.0.
• Cleans stale newer 1.3.x-only packages from python_modules during setup so local runs cannot pass due to leftovers.
• Makes Worker create_agent integration tests fail on 501 instead of skipping when create_agent is unavailable.

Why

A separate Dependabot security-update job on main failed because the Worker example declared broad direct langchain>=0.3.0,<1.0.0 without a lockfile/pinned installed version.

The correct Worker fix is not to move pyproject.toml to newer langchain>=1.3.x: that breaks pywrangler sync and Pyodide because newer transitive dependencies pull native packages without Pyodide wheels. The Worker example already uses a manual vendoring script for this reason, so this makes that split explicit and keeps create_agent supported.

Validation

• cd libs/langchain-cloudflare/examples/workers && env PATH="$HOME/.nvm/versions/node/v22.17.1/bin:$PATH" uv run pywrangler sync --force - passed
• cd libs/langchain-cloudflare/examples/workers && env PATH="$HOME/.nvm/versions/node/v22.17.1/bin:$PATH" ./scripts/setup_pyodide_deps.sh - passed
• cd libs/langchain-cloudflare && set -a && source ../../.env && set +a && env PATH="$HOME/.nvm/versions/node/v22.17.1/bin:$PATH" uv run pytest 'tests/integration_tests/test_worker_integration.py::TestWorkerAgentTools::test_agent_tools[@cf/qwen/qwen3-30b-a3b-fp8]' 'tests/integration_tests/test_worker_integration.py::TestWorkerAgentStructuredOutput::test_agent_structured_output[@cf/qwen/qwen3-30b-a3b-fp8]' 'tests/integration_tests/test_worker_integration.py::TestWorkerAgentStructuredJsonSchema::test_agent_structured_json_schema[@cf/qwen/qwen3-30b-a3b-fp8]' -v -s - 3 passed
• cd libs/langchain-cloudflare && make lint - passed
• cd libs/langchain-cloudflare && make test - 110 passed, 2 skipped
• pre-commit run --all-files - passed

Note: representative Llama agent-tools and JSON-schema agent paths passed, while the Pydantic structured agent path hit an upstream Workers AI 504 Gateway Time-out. The same Pydantic path passed immediately on Qwen.