feat: support PureQL specification 0.1.0-preview.2.0.1

[kudima03]

Summary

PureQLQueryDocumentTransformer could not transform the current PureQL specification (0.1.0-preview.2.0.1): it threw NullReferenceException inside Microsoft.OpenApi. Since 0.1.0-preview.1.0.0 the specification keeps its definitions in a flat draft 2020-12 $defs (add.decimal@row, declares.integer.nullable, …) instead of the nested definitions tree the transformer was written for, and it uses constructs Microsoft.OpenApi cannot read back. This PR makes the transformer produce a correct OpenAPI document from the current specification, keeps support for the older definitions layout, and tests it against the real specification, pinned as a submodule.

Equivalence was checked end to end: the transformed components, turned back into a JSON Schema, accept all 96 valid queries in samples/valid and reject all 1636 in samples/invalid with ajv, exactly like the original specification. Specifications 0.1.0-preview.0.5.0, 1.0.0, 2.0.0 and 2.0.1 all transform with no dangling $ref.

Changes

• Read the flat $defs of current specifications: each entry becomes a component and #/$defs/... references are rewritten to #/components/schemas/.... The nested definitions tree of earlier specifications is still supported.
• Map characters not allowed in OpenAPI component names to _ (add.decimal@row → add.decimal_row).
• Drop $id and version from the Query schema along with $schema. Otherwise $id rebases every component $ref onto the specification's release URL.
• Spell boolean subschemas ("right": true, "then": true) as {} / { "not": {} }. Microsoft.OpenApi reads them as null and throws.
• Spell a non-string const as a one-value enum. Microsoft.OpenApi stores const as a string, which turned "const": true into "const": "True" and made valid queries invalid. A const next to an existing enum joins it through allOf, because an empty enum would be dropped.
• Add the PureQL-Specification submodule at src/Tests/PureQL-Specification, pinned to tag 0.1.0-preview.2.0.1 (same layout as PureQL.CSharp.Model.Serialization), and check out submodules in the build-and-test workflow.
• Add SpecificationTests against the real specification: every definition is emitted, component names are valid, no reference dangles or points outside the components, and boolean consts stay boolean.
• Add FlatDefinitionsTests covering $defs naming, reference rewriting, metadata stripping, boolean subschemas in every position, and const handling.
• Set the CI gates to the levels measured on this branch: COVERAGE_THRESHOLD 0 → 40 (line rate 40.83%, 265 / 649) and MUTATION_THRESHOLD 0 → 64 (mutation score 64.96%).
• Update the README to describe the $defs / definitions handling and the normalizations.

Screenshots

Not applicable (no UI changes).

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
PureQL.CSharp.Model.OpenAPI.Schema	41%	34%	345	➖
Summary	41% (265 / 649)	34% (113 / 330)	345	➖

Minimum allowed line rate is 40%