Fix Roslyn step-definition discovery to honor Cucumber Expression alternation and optional text

[clrudolphi]

Summary

• StepDefinitionFileParser.BuildRegex treated / (alternative text) and () (optional text) as literal characters, so a Roslyn-discovered binding's regex diverged from the connector-discovered regex (computed by Reqnroll's real runtime) for any Cucumber Expression using those constructs.
• Now delegates to the real Cucumber.CucumberExpressions library (v17.1.0 — the exact version Reqnroll 3.2.0 itself depends on) via its public CucumberExpression class, the same one Reqnroll's runtime uses for this. This is option 2 from the original issue: LSP.Core's regex generation is guaranteed to agree with the connector-discovered path by construction, rather than by two independently-maintained implementations staying in sync. (An earlier revision of this PR hand-rolled the alternation/optional grammar instead — replaced once it became clear taking the dependency was straightforward.)
• Two small pieces the package itself doesn't provide were ported, not re-derived, from Reqnroll's own runtime source (verified against a local Reqnroll checkout):
  • CucumberExpressionDetector — the plain-regex-vs-Cucumber-Expression heuristic. This lives in the Reqnroll runtime assembly itself, not in Cucumber.CucumberExpressions, and LSP.Core deliberately doesn't reference that assembly so discovery can run pre-build.
  • LspCucumberExpressionParameterTypeRegistry — a minimal IParameterTypeRegistry using the exact same regex fragments as Reqnroll's own registry (ParameterTypeConstants). Reqnroll's real registry builds its parameter-type set by reflecting over already-compiled binding methods, which doesn't exist yet during syntax-only discovery; unknown names (custom [StepArgumentTransformation] types, enums) fall back to a permissive .* match instead of throwing, preserving today's behavior.
• Hardens BuildRegex against the real grammar's stricter validation (e.g. "an alternative may not be empty") throwing for malformed input the old hand-rolled version silently tolerated — caught and surfaced as an invalid (null-regex) binding rather than aborting discovery of the whole file.
• The existing "no {param} → treat as plain regex" contract is preserved via CucumberExpressionDetector, so hand-written regex bindings using bare (...) capturing groups keep working exactly as before.

Fixes #476

Test plan

• Unit tests in StepDefinitionFileParserTests covering: / alternation, () optional text, alternation+optional combined, backslash-escaped syntax, standard parameter types (now asserted by match behavior rather than exact regex text, since the pattern text is the library's own), {word} not matching across a space, a custom/unknown parameter type falling back to wildcard, and a malformed expression degrading to an invalid binding instead of throwing.
• dotnet test tests/LSP/Reqnroll.IdeSupport.LSP.Core.Tests — 673 passed, 0 failed, 1 skipped (pre-existing skip).
• Full solution build + full solution dotnet test — all green.

🤖 Generated with Claude Code