Migrate Mason framework templates to durable runtime

[shivam5]

Stacked dependency

This is a draft follow-up to #476. Review the single branch-only commit 555f5b7 after #476's 08b5e7a; once #476 merges, this PR's GitHub diff will collapse to only the template migration.

What changed

• Migrates the existing agent-langgraph and agent-openai templates to databricks_mason.DurableAgentApp.
• Keeps runtime/main.py as a thin app-construction layer and removes each template's duplicated runtime/runtime.py server.
• Adapts both chat UI overlays to client-generated invocation IDs, the single __Host-databricks-app-router cookie, durable polling, and persisted event replay.
• Configures mason deploy to provision/reuse the durability Lakebase backend for durability-app, agent-langgraph, and agent-openai projects.
• Removes the obsolete package-level in-memory runtime/background.py and its now-unused uuid-utils direct dependencies.
• Updates the E2E tool matrix to send stable client-generated IDs across retries.
• Keeps /api/healthz absent; the first invocation initializes the routing cookie locally.

The minimal durability-app introduced in #476 is unchanged by the branch-only diff.

Validation

• Mason unit tests: 349 passed
• LangGraph API-only generated project: 15 passed, 1 skipped
• LangGraph chat generated project: 23 passed, 1 skipped
• OpenAI API-only generated project: 13 passed, 1 skipped
• OpenAI chat generated project: 21 passed, 1 skipped
• All four generated projects build successfully
• ruff check
• ruff format --check
• ty check
• uv lock --check
• git diff --check
• Mason package build succeeds
• No healthz, uuid-utils, BackgroundRuns, or runtime.background references remain in Mason