Fix nil pointer panic in FormatOnEnter for if-else statements

[Copilot]

Fixes panic when pressing Enter in TypeScript files containing if-else statements. The panic occurred when FindPrecedingToken returned nil in edge cases, but the code only had a debug assertion (no-op in release builds) before dereferencing the pointer.

Changes

• Added nil check in childStartsOnTheSameLineWithElseInIfStatement: Returns false when FindPrecedingToken returns nil instead of dereferencing a nil pointer
• Added test coverage: TestFormatOnEnter_NilPrecedingToken validates the fix with multiple if-else edge cases

// Before: panic on nil in release builds
elseKeyword := astnav.FindPrecedingToken(sourceFile, child.Pos())
debug.AssertIsDefined(elseKeyword)  // no-op in release builds
elseKeywordStartLine, _ := getStartLineAndCharacterForNode(elseKeyword, sourceFile)

// After: safe handling of nil
elseKeyword := astnav.FindPrecedingToken(sourceFile, child.Pos())
if elseKeyword == nil {
    return false
}
debug.AssertIsDefined(elseKeyword)
elseKeywordStartLine, _ := getStartLineAndCharacterForNode(elseKeyword, sourceFile)

Original prompt

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This section details on the original issue you should resolve

<issue_title>panic handling request textDocument/onTypeFormatting</issue_title>
<issue_description>## Stack trace

panic handling request textDocument/onTypeFormatting runtime error: invalid memory address or nil pointer dereference goroutine 15626 [running]:
runtime/debug.Stack()
	runtime/debug/stack.go:26 +0x5e
github.com/microsoft/typescript-go/internal/lsp.(*Server).recover(0xc00019adc0, 0xc01a052ae0)
	github.com/microsoft/typescript-go/internal/lsp/server.go:571 +0x58
panic({0xb5c6a0?, 0x14bd140?})
	runtime/panic.go:783 +0x132
github.com/microsoft/typescript-go/internal/ast.(*Node).Pos(...)
	github.com/microsoft/typescript-go/internal/ast/ast.go:240
github.com/microsoft/typescript-go/internal/scanner.GetTokenPosOfNode(0x88e?, 0xc017b961a0?, 0x8?)
	github.com/microsoft/typescript-go/internal/scanner/scanner.go:2324 +0xb2
github.com/microsoft/typescript-go/internal/format.getStartLineAndCharacterForNode(0xc006fddd80?, 0xc01e97c608)
	github.com/microsoft/typescript-go/internal/format/indent.go:250 +0x1a
github.com/microsoft/typescript-go/internal/format.getIndentationForNodeWorker(0xebdd90?, 0xc01e97c608?, 0x7fe?, 0xc0081a7d38, 0xebf7a8?, 0xc01e97c608, 0x0, 0xc01653f1f0)
	github.com/microsoft/typescript-go/internal/format/indent.go:69 +0x1b0
github.com/microsoft/typescript-go/internal/format.GetIndentationForNode(0xc006fddd80, 0xc0081a7d38, 0xc01e97c608, 0xc01653f1f0)
	github.com/microsoft/typescript-go/internal/format/indent.go:17 +0x69
github.com/microsoft/typescript-go/internal/format.FormatSpan({0xebf7a8, 0xc01370f4a0}, {0x81b, 0x833}, 0xc01e97c608, 0x2)
	github.com/microsoft/typescript-go/internal/format/api.go:69 +0x105
github.com/microsoft/typescript-go/internal/format.FormatOnEnter({0xebf7a8, 0xc01370f4a0}, 0xc01e97c608, 0x0?)
	github.com/microsoft/typescript-go/internal/format/api.go:184 +0x20c
github.com/microsoft/typescript-go/internal/ls.(*LanguageService).getFormattingEditsAfterKeystroke(0xc000e09248?, {0xebf7e0?, 0xc017d16c80?}, 0xc01e97c608, 0x0?, 0x833, {0xee2e30, 0x1})
	github.com/microsoft/typescript-go/internal/ls/format.go:126 +0xf0
github.com/microsoft/typescript-go/internal/ls.(*LanguageService).ProvideFormatDocumentOnType(0xc01370f230, {0xebf7e0, 0xc017d16c80}, {0xc00a36a0e0?, 0x100000000000000?}, 0xc0096a41c0, {0xb4b2240?, 0xc0?}, {0xee2e30, 0x1})
	github.com/microsoft/typescript-go/internal/ls/format.go:78 +0x119
github.com/microsoft/typescript-go/internal/lsp.(*Server).handleDocumentOnTypeFormat(0xc0003a2a20?, {0xebf7e0?, 0xc017d16c80?}, 0xc00a36a0e0?, 0x70?)
	github.com/microsoft/typescript-go/internal/lsp/server.go:889 +0x4a
github.com/microsoft/typescript-go/internal/lsp.init.func1.registerLanguageServiceDocumentRequestHandler[...].22({0xebf7e0, 0xc017d16c80}, 0xc01a052ae0)
	github.com/microsoft/typescript-go/internal/lsp/server.go:557 +0x11a
github.com/microsoft/typescript-go/internal/lsp.(*Server).handleRequestOrNotification(0xc00019adc0, {0xebf7e0, 0xc017d16c80}, 0xc01a052ae0)
	github.com/microsoft/typescript-go/internal/lsp/server.go:451 +0x10d
github.com/microsoft/typescript-go/internal/lsp.(*Server).dispatchLoop.func1()
	github.com/microsoft/typescript-go/internal/lsp/server.go:356 +0x3a
created by github.com/microsoft/typescript-go/internal/lsp.(*Server).dispatchLoop in goroutine 19
	github.com/microsoft/typescript-go/internal/lsp/server.go:376 +0x9ad

Steps to reproduce

1. In a new project Just open a .ts/tsx file and press enter.

On latest preview build (7.0.0-dev.20251108.1), i never got this to work on any previous version, i know a similar issue was closed, but it didn't fix anything for me.</issue_description>

<agent_instructions>Create a failing test first, commit it, then fix the issue.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes panic handling request textDocument/onTypeFormatting #2042

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