test: rename handlers/*.test.go so the tests actually run

[manantlerio]

Summary

handlers/api.test.go, handlers/proxy.test.go and handlers/raw.test.go are
named with a dot rather than an underscore. go test only picks up
*_test.go, so none of these have ever been compiled or run as tests. They are
built into the package as ordinary source, which is also why testing and
net/http/httptest end up as imports of the non-test build.

Renaming them to *_test.go runs them for the first time. All four tests
failed, so this fixes them as well.

Every failure turned out to be in the test, not in the handler. No runtime
behaviour is changed in this PR.

What was broken

TestProxySite mounted the handler on /:url:

app.Get("/:url", ProxySite(""))

ProxySite reads c.Params("*"), and cmd/main.go mounts it on /*. A single
:url param cannot match https://example.com, so the request 404'd and the
handler was never reached at all. Now mounted on /* to match production.

TestApi / invalid url expected 400. Api returns 500.

TestRaw / invalid url expected status 200 and the body
parse invalid-url: invalid URI for request. Raw returns 500 and
Get "invalid-url": unsupported protocol scheme "", i.e. the error comes from
the HTTP client, not from url.Parse. The test also asserted StatusCode == 200
for every case, so it could not have expressed an error case even in principle.

TestRewriteHtml expected <img src= but rewriteHtml emits <img src=,
with two spaces. The replacement is "<img $1 src=" and $1 is empty when the
tag carries no other attributes.

Judgement calls

Two of these look like real (if minor) defects, and I have deliberately not
fixed them here, because this PR is about test naming and fixing them would
change behaviour:

1. A malformed target URL returns 500. Arguably it should be 400, since it
   is the caller's input that is wrong. Changing it is an API change. Happy to
   send it separately if you want it.
2. rewriteHtml emits a double space in <img src= and <script src=.
   Harmless in HTML, but it is sloppy output and the original test author
   clearly expected a single space. Fixing it properly means restructuring the
   capture group so the whitespace lives outside it, which is a change to output
   on every proxied page. Also happy to send separately.

Both are asserted as current behaviour with a comment saying so, so the tests
document reality rather than blessing it silently.

Also

The two "valid url" cases pointed at www.google.com. I switched them to
example.com, which is reserved by IANA for exactly this purpose and is 559
bytes rather than a full search page.

They still require network access, so the suite is not hermetic. Converting them
to httptest servers would fix that and is worth doing, but it interacts with
#153 (which blocks fetches to private addresses, including loopback), so it
wants to be a deliberate follow-up rather than a drive-by here.

Verification

go vet, gofmt, go build ./... clean and go test ./... green, in
golang:1.26:

ok  ladder/handlers      0.085s
ok  ladder/pkg/ruleset   1.134s

Independent of #153, which touches different files.