Align Create Agent to canonical [full_text] content shape

[hiydavid]

Part of epic #173 (Full Unification of GSL Instruction Schema).

Goal

Create Agent currently splits text_instructions content per-line:

content: ["## Terminology\n", "- item1\n", ...]

GSO stores as a single-item list content: [full_text] because Genie UI only renders newlines correctly with one string (see comment in packages/.../optimization/applier.py L204–206: "Stores the full text as a single content list item so that the Genie UI renders newlines correctly. The content field is list[str]; using multiple items causes the UI to concatenate them without line breaks.").

Migrate Create Agent to the canonical shape.

Acceptance criteria

• backend/services/create_agent_tools.py::generate_config L2047–2052 stops the per-line \n split; produces content: [full_text]
• backend/references/schema.md L58–66 example rewritten to single-item shape
• Regression test in backend/tests/test_genie_creator.py asserts the new shape passes through _enforce_constraints unchanged
• Manual QA: create a space via Create Agent, render in Databricks Genie UI, confirm newlines render correctly

Dependencies

• Related to Align create agent output to GSL schema #89 (near-term Create Agent work ships the prompt vocabulary change; this ticket ships the shape change)