[testify-expert] Improve Test Quality: pkg/cli/codemod_copilot_web_fetch_test.go #74912

1. Missing / high-value tests

• isNestedWorkflowFragment has no direct test. Add a table test: "", .github/workflows/a.md (false), .github/workflows/shared/a.md (true), workflows/a.md (false), a/b.md without workflows (false), Windows-style separators, paths with ...
• getCopilotWebSearchRemovalCodemod metadata (ID copilot-web-search-removal, IntroducedIn) and the web-search branch of applyCopilotWebToolRemoval are only covered in codemod_copilot_web_search_test.go; add a shared table over both codemods (tool key param) to guarantee parity.
• Error path: Apply with malformed frontmatter / tools of non-map type should return unchanged content without error (assert explicitly).
• Non-Copilot engine (e.g. claude) with web-fetch: true must be left unchanged.

2. Testify assertion upgrades

• Metadata test: use assert.NotNil -> require.NotNil before invoking (already done) and add assert.Equal on Name/Description non-empty via assert.NotEmpty (present) — consider also asserting ID uniqueness against the registry (assert.Len of IDs set).
• Use assert.Equal(t, updated, updatedAgain) -> assert.Equal is fine; prefer assert.YAMLEq only for frontmatter fragments, keep exact string equality for full-file output.
• Add messages to key require.True(t, applied, "codemod should apply") for easier failure diagnosis.

3. Table-driven refactors

_CompatibleInputsAreUnchanged and _YAMLFormsAndLiteralText (lines ~67-220) are candidates for []struct{name, content, frontmatter, want, wantApplied} with t.Run(tt.name, ...) and t.Parallel() per subtest.

Before:

updated, applied, err := codemod.Apply(content, fm)
require.NoError(t, err)
assert.False(t, applied)

After:

for _, tt := range tests {
    t.Run(tt.name, func(t *testing.T) {
        t.Parallel()
        got, applied, err := codemod.Apply(tt.content, tt.fm)
        require.NoError(t, err)
        assert.Equal(t, tt.wantApplied, applied)
        assert.Equal(t, tt.want, got)
    })
}

4. Organization / readability

• Extract a small helper for building engine/tools frontmatter maps to cut repeated literals.
• Name tests by behavior (e.g. _RemovesWebFetchWithCopilotEngine).
• Group file-resolution tests (ResolvesIncludedEngine, PreservesSettingWhenEngineResolutionFails) using shared temp-dir setup helper.