Coder's rate limiter keyed its bucket on the raw, un-normalized request
path (`httprate.KeyByEndpoint` reads `r.URL.Path` directly). The
router's `singleSlashMW` already collapses redundant slashes so a
request like `/api/v2/users//validate-password` reaches the same handler
as the canonical path, but it never touched `r.URL.Path`, so the rate
limiter saw a different key and let a client bypass a limit it had
already hit just by respelling the URL.

`keyByNormalizedEndpoint` replaces `KeyByEndpoint` and runs `path.Clean`
on `r.URL.Path` before using it as the key, so equivalent paths share
one bucket. Includes a unit test at the key-function level and an
integration test (`TestRateLimitPathNormalization`) that reproduces the
bypass against a real server.

Fixes CDM-02-003 (Cure53). Refs
https://github.com/coder/security-disclosures/issues/166.

(cherry picked from commit de716f8)