test: add coverage for fetch_indexes_from_main

[darthmelonder]

Summary

• Adds a TestFetchIndexesFromMain class in tests/test_doc_index.py covering fetch_indexes_from_main() in src/doc_index.py.
• Nine test methods across the happy path and every failure mode the function handles: git fetch failure, no index tracked on the base branch (empty ls-tree stdout), ls-tree non-zero return, git checkout failure, and unexpected exceptions.
• Also covers configuration paths: custom DOCS_BASE_BRANCH, empty DOCS_BASE_BRANCH fallback to main, and DOCS_SUBFOLDER producing prefixed index paths (docs/.doc-index).
• Matches the mocking style of the adjacent TestCheckoutDocsFromBaseBranch class — uses patch("doc_index.run_command_safe") with a class-level _mock_git helper that returns a side_effect simulating the three git commands.
• The _mock_git helper raises AssertionError on any git command it doesn't explicitly handle, so future refactors that introduce a new git call will fail the tests loudly rather than silently pass with a benign default.
• Verifies the diagnostic log strings introduced by fix: improve index fetch diagnostics and use targeted ls-tree #41 ("No cached indexes/summaries found", "Could not fetch", "Could not checkout indexes", "Error fetching indexes from main") via capsys, so a regression in the diagnostic output is caught.

Closes #42

Test plan

• All 393 tests pass (was 384 before this change — 9 new tests added, no existing test modified).
• Coverage remains above the 60% floor (currently 65.59%).
• uv run ruff check src/ tests/ — clean.
• uv run ruff format --check src/ tests/ — clean.
• Fail-fast helper verified: no existing test path triggers the unexpected-command assertion.