Consolidate URLSession blog articles into one URL

[AvdLee]

Summary

We had two blog articles competing for the same "debug/monitor URLSession requests without a proxy/certificates" topic cluster:

• Old (Oct 2025): /blog/monitor-urlsession-network-requests-without-the-pain-of-custom-certificates/ — ranks well: GSC position 5.6, 238 impressions over 2026-07-16..2026-08-15
• New (Jun 2026): /blog/debug-urlsession-requests-without-proxy/ — failed to rank: position 6.8, only 43 impressions and 1 click over the same window

Rather than keep splitting relevance signals across two URLs, this PR consolidates them into the URL Google already prefers.

Changes

• Merged the newer article's modern sections (cURL export, Networking Insights / duplicate calls, AI network request prompts, honest proxy-tool positioning) into the ranking article, with updated modifiedTime, meta description, and hero image frontmatter.
• Deleted docs/src/content/blog/debug-urlsession-requests-without-proxy/ (its images live in the shared docs tree and are now referenced by the merged article).
• Added a redirect from /blog/debug-urlsession-requests-without-proxy to the ranking URL in astro.config.ts.
• The old article's title and slug are intentionally unchanged to avoid confounding the site-wide SEO titles experiment currently running.

RSS and llms.txt are generated at build time, so no manual updates were needed. The only remaining reference to the old new-slug is the redirect entry.

Validation

All run from docs/:

• npm run lint ✅
• npm run format:check ✅
• npm run typecheck ✅ (0 errors, pre-existing hints only)
• npm run build ✅ (91 pages)
• npm run knip ✅
• npm test ✅ (7/7 pass)

Verified in dist/: the redirect stub page exists for /blog/debug-urlsession-requests-without-proxy/ (meta refresh + noindex + canonical to the ranking URL), and the merged article contains the new sections ("Copy a failing request as cURL", "Networking Insights") with a self-referencing canonical.