prefetch: don't drop replacements for non-participating optional capture groups

[moonchen]

Problem

Pattern::replace() in the prefetch plugin rejects any replacement reference $N whose index is >= the value returned by the match. But that value is one past the highest capture group that participated in the match, not the number of groups the pattern defines. A trailing optional group such as (\?.*)? that does not participate — e.g. a request with no query string — lowers that count and makes a perfectly valid $N look out of range.

Concretely, with a remap like:

@plugin=prefetch.so @pparam=--fetch-path-pattern=/(.*-)(\d+)(\?.*)?$/$1{$2+1}$3/

every matching request without a query string logs:

ERROR: (prefetch) invalid reference in replacement string: $3

and the prefetch is silently dropped (the client is still served).

Fix

• replace(): at match time, substitute an empty string for a group that did not participate instead of failing the whole replacement — the documented PCRE2 semantics for an unmatched group.
• compile(): validate $N references once, at config-load time, against the pattern's actual capture-group count (Regex::get_capture_count()) — the correct place to catch a genuinely out-of-range reference such as $5 against a 3-group pattern, and a pattern defining more groups than can be captured ($0..$9).

Additional hardening

• Treat an unusable --fetch-path-pattern, and invalid --fetch-count / --fetch-max / --fetch-overflow values, as configuration errors: the remap instance fails to load instead of silently running with prefetch disabled. On a running server, traffic_ctl config reload safely rejects the bad config and keeps the current one.
• Skip an empty expanded path instead of scheduling a zero-length fetch (which BgFetch turns into a self-prefetch of the original request path).
• Remove the now-dead Pattern::process() / Pattern::capture() helpers (no callers).

Note for reviewers: the "refuse to load on invalid config" behavior change is intentional and aligns with how --fetch-policy already behaves, but it is a behavior change — happy to split it into a separate PR if preferred.

Tests

Adds three autests under tests/gold_tests/pluginTest/prefetch/:

• prefetch_optional_group — the non-participating optional-group case (fails on the pre-fix code).
• prefetch_bad_pattern_refused — an over-limit (10-group) pattern makes ATS refuse to load the remap.
• prefetch_empty_replacement — an empty-collapsing replacement is logged and skipped, not self-prefetched.

Built locally (macOS, PCRE2); all prefetch autests pass. Draft pending CI.