Add host-owned ASGI route registration

[dcaayushd]

Summary

Fixes #447.

Adds a public lifecycle split for host-owned ASGI integrations:

• Adds App.register_routes() to synchronously register the Teams messaging endpoint without starting a server or running async plugin init.
• Adds App.start_plugins() so hosts can run plugin on_init hooks later from their ASGI startup lifecycle.
• Returns a public HttpRoute route table from registration.
• Ships StarletteAdapter as part of microsoft_teams.apps.http instead of leaving it example-only.
• Updates host-owned FastAPI/Starlette examples and docs to use the new lifecycle.

Validation

./.venv/bin/python -m pytest packages/apps/tests packages/botbuilder/tests
./.venv/bin/python -m ruff check packages/apps/src/microsoft_teams/apps packages/apps/tests/test_app.py packages/apps/tests/test_http_server.py examples/a2a/src/main.py examples/http-adapters/src/fastapi_non_managed.py examples/http-adapters/src/starlette_echo.py examples/mcp-server/src/main.py
./.venv/bin/pyright packages/apps/src/microsoft_teams/apps packages/botbuilder/src/microsoft_teams/botbuilder/adapter.py examples/a2a/src/main.py examples/http-adapters/src/fastapi_non_managed.py examples/http-adapters/src/starlette_echo.py examples/mcp-server/src/main.py