fix(fetchers/session): fallible client build + typed SessionError (#29 #45)

[Liohtml]

Summary

Error-handling hardening for the fetch path (both Medium):

• [repo-health] Medium: FetcherClient::new() panics on TLS initialization failure #29 — Fetcher::new panicked on client build failure: build_client ended in .expect("Failed to build reqwest client"), so a TLS-backend init failure crashed the process. build_client and Fetcher::new now return Result<_, FetcherError>. The fallibility propagates through SessionManager::add_session / ensure_default and CrawlerEngine::new; the CLI reports the error and exits cleanly.
• [repo-health] Medium: SessionManager::fetch() erases error type — session-not-found and network failures are indistinguishable #45 — SessionManager::fetch erased the error type: it returned Result<_, String>, conflating "session not found" (config bug, unrecoverable) with network failures. Introduced SessionError { NotFound, UnsupportedMethod, Network }. Callers can now match on the variant; the engine still forwards the message to on_error.

Test plan

• Updated spiders_session and fetchers_client tests for the Result-returning signatures
• cargo test, clippy --all-targets -D warnings, fmt --check green
• CrawlerEngine::new has no in-tree callers, so the signature change is contained

Closes #29
Closes #45

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Generated by Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 53af5826-8667-4234-9aa6-65ae3f2b67a0

📥 Commits

Reviewing files that changed from the base of the PR and between eb19302 and 097d771.

📒 Files selected for processing (6)

• src/fetchers/client.rs
• src/main.rs
• src/spiders/engine.rs
• src/spiders/session.rs
• tests/fetchers_client.rs
• tests/spiders_session.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch claude/fetcher-session-errors

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