Add a smoke test for the `AVAILABLE_MODELS` schema

[DevanshSrajput]

Context

The existing AppHelpersModuleTests already asserts the
catalogue has no banned ids, every entry has the required
fields, and list_model_choices() orders Free first. A
new contributor can still introduce subtle bugs: duplicate
ids, multiple Free entries marked as the default, a tier
value that's neither "Free" nor "Paid", or a performance
rating that breaks the existing 3-star scheme. A single
test_catalogue_is_well_formed smoke test catches all of
these in one go.

What to change
Add one test method to AppHelpersModuleTests in
tests/test_agent.py that asserts:

• No two entries in AVAILABLE_MODELS share a name field.
• tier is one of {"Free", "Paid"} for every entry.
• performance matches r"^⭐+$" (one or more star emojis).
• description is non-empty for every entry.
• Exactly one entry has tier == "Free" AND is the
  DEFAULT_MODEL_ID (locks the "implicit default" contract
  from G9).

How to verify

• The new test passes on the current catalogue.
• Mutating the catalogue to break any rule makes the test
  fail with a precise error message.
• python -m unittest tests.test_agent.AppHelpersModuleTests -v
  is green.

Skill: schema validation, fixture design.
Estimated effort: S.