fix(indexes): stop section extraction at unnumbered headings and allow EOF intent extraction

[mjmirza]

Fixes two section boundary parsing issues in index generator tools:

1. In tools/gen-by-problem-by-language.py, extract_section used lookahead regex (?=^##[#]?\s+\d+\.|\Z) requiring a digit and dot. When an unnumbered heading like ## Code examples followed a section, extraction overshot past the section boundary into code blocks. Updated lookahead to (?=^##\s+|\Z) so section extraction terminates at any top-level ## heading while permitting ### subheadings.

2. In tools/gen-indexes.py, first_intent used lookahead (?=^##\s) without |\Z, failing to extract Section 2 intent when Section 2 was at EOF. Updated lookahead to (?=^##\s|\Z).

3. Added unit tests in tools/gen-by-problem-by-language-test.py and tools/gen-indexes-test.py, and updated generated pathway index docs/BY-LANGUAGE.md.

---

PR created automatically by Jules for task 11736323918473114439 started by @mjmirza

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.