Closes DEVEX-604

Hardens `redirect` URL handling in the OAuth2/OIDC/external-auth
callback flows so redirects are always reduced to a safe, relative path
local to the application. Previously a redirect value with an opaque
scheme (e.g. `javascript:...`) or a path with multiple leading slashes
(e.g. `///evil.com`) could survive sanitization mostly intact.

Also de-duplicates the previously copy-pasted `uriFromURL` helper (now
exported `httpmw.URIFromURL`) so there's a single implementation shared
by `coderd/userauth.go`, `coderd/externalauth.go`, and
`coderd/httpmw/oauth2.go`.

<details>
<summary>Context</summary>

Addresses a low-severity finding reported via a pentest disclosure: the
redirect sanitizer used `url.Parse(...).RequestURI()`, which doesn't
reject non-hierarchical (opaque) URLs and doesn't collapse extra leading
slashes, so crafted `redirect` values could partially survive
sanitization.

</details>

This PR was authored by a Coder Agent on behalf of @aslilac.

(cherry picked from commit 2f87991)