fix(completions): exclude default keyword from expression-body arrow function completions

[tsushanth]

Fixes #63463

Problem

default appeared as a keyword completion inside expression-bodied arrow functions:

(d, defaultIsAnInvalidParameterName) => d
//                                       ^ `default` offered here — invalid

Two existing fourslash tests already acknowledged this with TODO and "probably stop working" comments.

Root cause

getRelevantTokens sets contextToken to the => token (the preceding token before the identifier/keyword being typed). Inside tryGetFunctionLikeBodyCompletionContainer, findAncestor starts at contextToken.parent (the ArrowFunction) with prev = undefined, so the check prev === node.body never matches on the first iteration. The function returns undefined, and getGlobalCompletions falls back to KeywordCompletionFilters.All, which includes every isFunctionLikeBodyKeyword hit — and default passes that test because it is neither a contextual keyword nor a class-member keyword.

Fix

1. Detect the expression-body => context directly in tryGetFunctionLikeBodyCompletionContainer: when contextToken is EqualsGreaterThanToken whose parent is an ArrowFunction with a non-block body, return that arrow function as the container.

2. Add FunctionLikeExpressionBodyKeywords filter that delegates to isFunctionLikeBodyKeyword but excludes DefaultKeyword.

3. In getGlobalCompletions, use the new filter when the detected container has a non-block body, so default is omitted only in expression-body contexts. Block bodies (including switch/default: inside functions) are unaffected.

4. Update the two fourslash tests to assert excludes: ["default"] instead of including the TODO/known-bad expectation.

Tests

• completionListAtEndOfWordInArrowFunction02.ts — updated, passes
• completionListAtEndOfWordInArrowFunction03.ts — updated, passes
• All 307 completionList* fourslash tests pass
• completionListKeywords.ts (global scope default) continues to pass

[Copilot]

Pull request overview

This PR fixes a keyword-completion edge case in the language service where default was incorrectly suggested within expression-bodied arrow functions, by refining how the completion context is detected and applying a more appropriate keyword filter for concise arrow bodies.

Changes:

• Teach tryGetFunctionLikeBodyCompletionContainer to recognize the => token context for expression-bodied arrows and treat the containing ArrowFunction as the completion container.
• Add a new keyword filter (FunctionLikeExpressionBodyKeywords) that excludes default while keeping existing block-body behavior unchanged.
• Update two fourslash tests to assert "default" is excluded (removing prior TODO/known-bad expectations).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/services/completions.ts	Detect expression-bodied arrow => context and apply a new keyword filter that omits default only for concise bodies.
tests/cases/fourslash/completionListAtEndOfWordInArrowFunction02.ts	Update completion assertions to exclude "default" in expression-bodied arrow completion context.
tests/cases/fourslash/completionListAtEndOfWordInArrowFunction03.ts	Update completion assertions to exclude "default" (matching the corrected keyword filtering behavior).