[Repo Assist] fix: emit Keyword semantic token for null in nullable type annotations

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Closes #1381

Problem

The null keyword in F# 9 nullable type annotations (e.g. string | null) was not receiving a semantic token classification. This caused editors to display null without keyword highlighting in these positions, even though null elsewhere in code does receive keyword highlighting.

Root Cause

FCS.GetSemanticClassification does not emit tokens for the null keyword in SynType.WithNull positions — it handles the | null portion of the type through the type-checking phase differently, and the resulting keyword token is simply absent from the emitted classification list.

Fix

Walk the untyped parse tree (ParseTree) for SynType.WithNull nodes and emit Keyword semantic tokens for each null occurrence found. These are collected in UntypedAstUtils.NullableTypes.collectNullKeywordRanges and appended to the FCS-provided tokens inside handleSemanticTokens in AdaptiveFSharpLspServer.fs.

Implementation details:

• Added module NullableTypes in UntypedAstUtils.fs with a SyntaxCollectorBase subclass that walks SynType.WithNull nodes and computes the null keyword range from the full type range (the null keyword is always exactly the last 4 characters of the WithNull full range).
• Added the corresponding signature in UntypedAstUtils.fsi.
• handleSemanticTokens calls collectNullKeywordRanges and appends (lspRange, Keyword, []) entries; if a range filter is active, only tokens within that range are included.

Trade-offs

• The fix uses the untyped AST, so it fires on parse without type-checking. This is consistent with how other semantic token supplements work in FSAC and avoids any perf regression on the type-checking path.
• The position of null is computed as the last 4 characters of the SynType.WithNull full range. This is reliable because F# syntax requires null to be spelled exactly (no aliases), but it does depend on the FCS SynType.WithNull full range including the null keyword and no trailing whitespace — which is the current behaviour.

Test Status

✅ Build: dotnet build — succeeded with 0 warnings, 0 errors.

✅ Tests: dotnet test -f net8.0 ./test/FsAutoComplete.Tests.Lsp/FsAutoComplete.Tests.Lsp.fsproj --filter Highlighting — 16/16 passed (including the new (28u, 26u) Keyword assertion for null in let withNull (x: string | null) = ()).

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[baronfel]

/repo-assist add a test that validates that the invariant about Null being the last 4 chars in the range is held, so that we are protected if the FCS behavior changes

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Commit pushed: 6ecf638

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[github-actions]

🤖 This is an automated response from Repo Assist.

Done! I've added a new FcsInvariantTests module to CoreUtilsTests.fs (commit 63ac035).

What the tests do

Rather than directly subclassing SyntaxCollectorBase (the .fsi exposes it as fully abstract, preventing object expressions), the tests call NullableTypes.collectNullKeywordRanges on parsed ASTs and then extract the actual source text at each returned range, asserting it equals "null".

This is the right level to test the invariant: if FCS ever changes SynType.WithNull.fullRange (e.g. adds trailing whitespace, or stops including the null keyword at the end), the computed ranges will shift and these tests will fail loudly.

Four test cases

Test	Source snippet
Single nullable parameter	let f (x: string | null) = ()
Multiple nullable parameters	let f (x: string | null) (y: string | null) = ()
Nullable on a non-first line	let f\n (x: string | null) = ()
Nullable return-type	let f () : string | null = null

All 4 pass locally:

EXPECTO! 4 tests run in 00:00:02.19 – 4 passed, 0 ignored, 0 failed, 0 errored. Success!

Generated by Repo Assist for issue #1466

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