Limit regex remap substitutions

[bneradt]

Regex remap targets could repeat valid substitution markers enough times
to exceed the fixed substitution arrays, even when every marker referred
to an allowed capture group.

This rejects targets with more substitution markers than the parser can
store and covers the boundary with remap parser unit tests.

[ezelkow1]

[approve ci]

[Copilot]

Pull request overview

This PR hardens regex remap parsing by enforcing an upper bound on the number of substitution markers ($0..$9) allowed in a regex remap target, preventing overflow of the fixed-size substitution arrays in UrlRewrite::RegexMapping. It also adds unit tests that exercise the boundary condition (max allowed vs. one over the limit).

Changes:

• Reject regex remap targets whose host template contains more than UrlRewrite::MAX_REGEX_SUBS substitution markers.
• Add unit tests validating that parsing succeeds at the limit and fails when exceeding it.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/proxy/http/remap/unit-tests/test_RemapRules.cc	Adds boundary unit tests for regex remap substitution marker count.
src/proxy/http/remap/RemapConfig.cc	Adds a guard to fail parsing when substitution markers exceed MAX_REGEX_SUBS.