fix: two silently-inert capabilities found by building a service on 26.07.01 (release 26.09.01)

[ancongui]

Two defects found while building a production service (dworkers) against 26.07.01. Both are the same
shape: a capability PyFly advertises, wired in a way that silently did nothing.

Fixed

The OTLP endpoint is now built to the OpenTelemetry specification.
OTEL_EXPORTER_OTLP_ENDPOINT is a base url — the SDK appends the per-signal path — while
OTEL_EXPORTER_OTLP_TRACES_ENDPOINT and the exporter's own endpoint= argument are the complete url,
used verbatim. TracingAutoConfiguration read the base variable and handed it straight to
OTLPSpanExporter(endpoint=...), collapsing the two: an operator who set the spec-correct
http://collector:4318 got an exporter POSTing to http://collector:4318, which is not a signal endpoint,
so every span was dropped and nothing was logged. The only way to make it work was to write a value into
the base variable that the spec says is not a base.

Both spellings work now — a url with no path is treated as a base and gains /v1/traces, one that already
has a path is left alone — and the same normalisation applies to pyfly.observability.tracing.otlp.endpoint.

@sse_mapping routes appear in the OpenAPI document.
collect_route_metadata() looked only at __pyfly_mapping__, so server-sent-event routes were absent from
/openapi.json and from pyfly openapi with nothing said about it. A CI job that exports the document and
diffs it — the standard way to keep an HTTP surface honest — could not see the streaming half of the API at
all, and a deleted stream read as no change. SSE is plain HTTP, so it is now emitted as the GET it is, with a
text/event-stream success response.

Added

x-pyfly-websocket-routes. WebSocket has no OpenAPI representation — that is what AsyncAPI is for — but
leaving @websocket_mapping routes out entirely made the document quietly incomplete in the same way SSE
was. ControllerRegistrar.collect_websocket_routes() reports them and the generator publishes them under
this document-level extension: still not operations, but visible, diffable, and honest about what the
document does not cover. Wired through pyfly openapi, the Starlette adapter and the FastAPI adapter alike.

opentelemetry-exporter-otlp-proto-http as a dev dependency. OTLP is the exporter
TracingAutoConfiguration selects by default as soon as an endpoint is configured, yet it appears in no
extra — so no CI job ever imported it and the entire OTLP path, the endpoint bug above included, went
unexercised. Dev-only: applications still choose and pay for their own exporter.

Changed

The README version badge, which the v26.07.01 release left at 26.06.114.

Verification

Gate	Result
pytest tests/ (CI's ignore set)	4998 passed, 7 skipped, 0 failed
mypy src/pyfly --strict	no issues in 694 source files
ruff check / ruff format --check	passed / 1274 files formatted
uv build	sdist + wheel built

Version bumped to 26.09.01 (PEP 440 26.9.1) with a CHANGELOG entry. The tag is deliberately not
pushed — pushing v* publishes, and that is a human decision.