rewrite: prevent placeholder re-expansion in injected query (#7761)

When the rewrite URI template ends with a literal '?' and contains a placeholder that expands to client-controlled bytes (e.g. {http.request.header.X-Fwd}), those bytes flow into buildQueryString which runs a second Replacer pass. If the bytes contain placeholder syntax such as {env.SECRET}, that placeholder is evaluated, allowing disclosure of environment variables, files (via {file./path}), or internal request vars through the rewritten request URI.

Escape '{' and '}' in the injected query before assigning it to the query variable, so the second pass cannot find any placeholder syntax to evaluate. Operator-written placeholders in the rewrite template are already expanded by the first pass on the path component, so the only '{' or '}' surviving into the injected query must have come from replacement values.

Fixes GHSA-j8px-rmrx-76h9.

Includes three regression tests mirroring the 'is not re-expanded' tests in modules/caddyhttp/vars_test.go.

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>