fix: anchor litestar trace-exclusion patterns to the normalized path

[lesnik512]

Closes #248.

The defect

opentelemetry_generate_health_check_spans=False silently did nothing on Litestar. Litestar
normalizes the trailing slash out of scope["path"], so OpenTelemetryMiddleware built
http://host/custom-health while the excluded entry was /custom-health/. ExcludeList
regex-searches, missed, and the health check was traced anyway.

The default health_checks_path is /health/, so this is the default configuration. FastAPI is
unaffected because Starlette does not normalize the slash away.

Confirmed before the fix with an in-memory exporter: 3 spans for /custom-health/ and 3 for
/custom-metrics/, each carrying http.url = http://testserver.local/custom-health.

The fix

Stripping the slash alone would have been wrong. ExcludeList.url_disabled is
bool(search(regex, url)) over a | alternation, so entries are unanchored substrings, and a bare
/custom-health would have started excluding /custom-healthy from tracing instead. So the derived
paths are rendered as anchored patterns:

rf"^\w+://[^/]*{re.escape(path.rstrip('/'))}(?:/|$)"

[^/]* cannot cross a slash, which pins the path to the start of the URL's path component, so
/api/custom-health does not match either. (?:/|$) admits sub-paths but not the lookalike. The
middleware's URL carries no query string (get_host_port_url_tuple builds scheme://host +
scope["path"]), so $ is safe.

Caller-supplied entries pass through verbatim

opentelemetry_excluded_urls entries are OpenTelemetry regexes, not paths. re.escape would have
destroyed an entry like client/.*/info, and anchoring it to the scheme and host would have broken
it a second way — on Litestar only, while FastAPI kept honoring it. They are therefore left
untouched, with a test pinning that.

That required separating the two kinds of entry, so _build_excluded_urls now composes
_build_infrastructure_excluded_paths() with the caller's list. FastAPI and FastStream consume the
merged set exactly as before. The exclusion policy — which paths, and the getattr sibling reads —
still lives in one method, so ADR-0002 holds; only the rendering differs per framework.

Scope

Litestar only, as the issue specified. FastAPI has the mirror-image defect from the same unanchored
search (prometheus_metrics_path="/metrics" also excludes /metrics-internal), but that is a false
exclusion rather than a false trace, nobody has hit it, and fixing it would change released behaviour
on a framework that currently works.

Verification

Span counts through an in-memory exporter, not url_disabled on a hand-written URL — the test added
in #247 asserted url_disabled("http://test/custom-health/"), a URL form the middleware never
receives, and passed while the behaviour it described was false. That stale comment is removed here.

• test_litestar_otel_excludes_infrastructure_paths_normalized_by_litestar — health and metrics
  paths emit no spans; /custom-healthy still does.
• test_litestar_otel_keeps_caller_supplied_excluded_urls_as_regexes — /items/\d+$ still excludes
  /items/42, which only passes if the regex survived unescaped.

Full suite green: 327 passed, three consecutive randomized runs. ruff format, ruff check --no-fix
and ty check clean.