Decouple Claude model location from discovery opt-in

[andy-xu-db]

ug claude --model-location main.default currently enables model discovery implicitly. Make the flags independent: --model-location adds Databricks-Model-Service-Parent-Schema, and --enable-model-discovery enables Claude Code's gateway model discovery. Use ug claude --model-location main.default --enable-model-discovery for discovery scoped to that schema.

Update CLI help and documentation, cover all four flag combinations, and verify that adding the schema header does not enable discovery. Stacked on #677; this PR targets andy/rename-parent-to-model-location.

Validation:

• The new flag-combination regression test failed on the parent branch for --model-location alone (1 failed, 3 passed), then passed with this change.
• 497 CLI, Claude agent, and integration-contract tests passed with the ambient ENABLE_SMART_ROUTING_V2 override unset.
• Ruff lint/format, type checking, git diff --check, and the integration runner's --help check passed.
• Live e2e/integration tests were not run locally.

Tests:

ug claude --model-location main.andy (only adds header but does not enable model discovery)

ug claude --model-location main.andy --enable-model-discovery (adds header and enables model discovery)