bug(web_fetch): cache TTL resolver logs spurious WARNING on every call when env unset + stale os.environ tests #11246

Problems (found via scoped unit-test triage)

web_fetch/cache.py _resolve_web_fetch_cache_ttl() reads config.misc.web_fetch_cache_ttl, a str SSOT field defaulting to "" (ssot_config.py:1562). Two issues:

1. Runtime bug — spurious warning: the guard is if raw is None, which does not catch the empty-string default. So when AUTOBOT_WEB_FETCH_CACHE_TTL is unset (the common case), the resolver hits int("") → ValueError → logs WARNING AUTOBOT_WEB_FETCH_CACHE_TTL='' is not an integer; falling back to 86400s on every call (and at import). Noise in normal operation.

2. Stale tests: tests/unit/web_fetch/test_cache.py TestTTLResolver/TestMaxBytesResolver patch os.environ, but the resolvers read config.misc.* (loaded once at import), so the override never takes effect — test_env_override (TTL + MaxBytes) fail even when run in isolation (2 failed, 20 passed); the other cases passed only by accident (config stayed ""/0 → default).

Fix

• Resolver: treat empty/None as 'use default' without warning (if raw is None or raw == ""); only a non-empty non-integer value warns.
• Tests: patch config.misc.web_fetch_cache_ttl / config.misc.web_fetch_max_bytes (the actual source) instead of os.environ; reframe the MaxBytes 'invalid string' case (impossible for an int field) to the zero-fallback path.

Acceptance

• No WARNING when the TTL env var is unset.
• test_cache.py genuinely exercises the resolvers via config; full file green.