Use full-file highlighting for diff sections (#36561)

* Fix #35252
* Fix #35999
* Improve diff rendering, don't add unnecessary "added"/"removed" tags for a full-line change
* Also fix a "space trimming" bug in #36539 and add tests
* Use chroma "SQL" lexer instead of "MySQL" to workaround a bug (35999)

services/gitdiff/gitdiff_excerpt_test.go

@@ -0,0 +1,39 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gitdiff
+
+import (
+	"bytes"
+	"strconv"
+	"testing"
+
+	"code.gitea.io/gitea/modules/translation"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestBuildBlobExcerptDiffSection(t *testing.T) {
+	data := &bytes.Buffer{}
+	for i := range 100 {
+		data.WriteString("a = " + strconv.Itoa(i+1) + "\n")
+	}
+
+	locale := translation.MockLocale{}
+	lineMiddle := 50
+	diffSection, err := BuildBlobExcerptDiffSection("a.py", bytes.NewReader(data.Bytes()), BlobExcerptOptions{
+		LeftIndex:     lineMiddle,
+		RightIndex:    lineMiddle,
+		LeftHunkSize:  10,
+		RightHunkSize: 10,
+		Direction:     "up",
+	})
+	require.NoError(t, err)
+	assert.Len(t, diffSection.highlightedRightLines.value, BlobExcerptChunkSize)
+	assert.NotEmpty(t, diffSection.highlightedRightLines.value[lineMiddle-BlobExcerptChunkSize-1])
+	assert.NotEmpty(t, diffSection.highlightedRightLines.value[lineMiddle-2]) // 0-based
+
+	diffInline := diffSection.GetComputedInlineDiffFor(diffSection.Lines[1], locale)
+	assert.Equal(t, `<span class="n">a</span> <span class="o">=</span> <span class="mi">30</span>`+"\n", string(diffInline.Content))
+}