Pin the measure schema block as a shared fixture, and fix the R array bug it exposes

[jat255]

Second of four stacked PRs building the Python semantic layer (M3). Based on #247.

search_pool shows the model one block per matching measure. That text is part of what the agent reads, so both packages must emit the same bytes. This adds measure_schema_text() and pins the format in tests/shared/measure-schema.json, with a runner in each suite.

Building the fixture exposed a live bug in the R package, which this PR also fixes. See the R section below for the detail. It is the only reason this PR touches pkg-r/.

Two Python defects surfaced the same way. An explicit heading="" was treated as absent where R passes it through, and that case is now in the fixture. And an argument typed with a bare enum.Enum, or any nullable argument, rendered as string with its vocabulary hidden from the model, because pydantic emits $ref and anyOf shapes the renderer did not handle.

The enum cases have Python-only tests rather than fixture cases. R has no counterpart to either: ellmer::type_enum() carries its values inline, and required = FALSE marks a type optional rather than producing a nullable union. There is no cross-language contract to pin.

Known gap, left deliberately: a multi-type union such as Region | OtherEnum | None still renders as string. No measure in the package constructs one, and a code comment records it.

R changes

arg_schema_line() renders one argument line of a measure's schema block. Its array branch called type_values() on the array's item type. type_values() reads the S7 values property, which ellmer::TypeEnum has and ellmer::TypeBasic does not. An array of a basic type therefore did not render poorly, it errored with Can't find property <ellmer::TypeBasic>@values. A new array_items_label() helper names a basic item type and still lists an enum item's vocabulary. The diff is five lines plus the helper.

Behaviour delta. Before, a measure with a string[] argument raised when its schema was rendered. Now it renders array of {string}. Enum arrays are unchanged byte for byte, because the helper reproduces the previous expression for that branch. Nothing else about the line moves.

Blast radius. arg_schema_line() has one caller, measure_schema_text(). That has one caller in the package, search_pool_text(), which backs the search_pool tool. So the only reachable effect is that a measure with a non-enum array argument can now appear in search_pool output instead of raising. Reaching it needs a @param with a string[]-style type code, which read_measures() parses into type_array(items = type_string()).

Why it is safe. The enum branch is textually the same expression as before, so no existing snapshot can move. The full package suite passes at 6649 tests with 0 failures. The new fixture runner in test-measures.R builds a measure from each shared case and compares the rendered text against it. It failed on exactly the string-array case before the fix and passes after it, which is the evidence that the fix targets the reported shape and nothing else.

What deserves scrutiny. Whether type_kind() is the right label for a basic item type. It returns the same words the scalar branch prints, so array of {string} reads consistently with (string, required), but a reviewer may prefer different wording. Worth a look because the fixture now pins it for both languages.

No hand-written R tests were deleted. pkg-r/tests/testthat/fixtures/shared/measure-schema.json is generated by scripts/sync-shared-fixtures.sh and needs no review.

[github-actions]

Preview deployed to Connect (dogfood.team.pct.posit.it): https://dogfood.team.pct.posit.it/connect/#/apps/d7a36cae-8f27-448b-a478-61b81fbe3942/draft/367000

Deployed from commit 6923b6a.

[github-actions]

Preview deployed to Connect (connect.staging.pct.posit.it): https://connect.staging.pct.posit.it/connect/#/apps/ad662e1b-5048-4acc-9ad7-f9478c92274e/draft/2461

Deployed from commit 6923b6a.