Fix #2951: drop the regex route constraint from the default Swagger route

[RaphaelFakhri]

Fixes #2951. This takes up the offer above: "We'd be happy to take a PR that changes the route template ... that doesn't need a Regex."

It still reproduces on master. SwaggerOptions.DefaultRouteTemplate is /swagger/{documentName}/swagger.{extension:regex(^(json|ya?ml)$)}, and SwaggerMiddleware's constructor eagerly resolves a TemplateBinder for it. An application built with CreateSlimBuilder or AddRoutingCore does not register the regex constraint, so a standalone app against master throws at startup with the exception in the report, from SwaggerMiddleware..ctor.

The default template drops the constraint, and the middleware applies the equivalent allow-list in code, but only when the extension parameter has no inline constraint of its own. A custom template that brings its own constraints keeps going through the TemplateBinder path unchanged. The allow-list is json, yaml, yml, compared with OrdinalIgnoreCase, matching the case-insensitivity of the regex it replaces.

MapSwagger's default parameter value changes with it, so PublicAPI.Unshipped.txt carries the *REMOVED* and replacement pair the analyzers require.

This also deletes the repo's own workaround: test/WebSites/WebApi.Aot/Program.cs was registering SetParameterPolicy<RegexInlineRouteConstraint> to get around exactly this, so the in-repo AoT slim-builder site now exercises the fix rather than papering over it.

SwaggerMiddlewareRouteTemplateTests covers building the pipeline under AddRoutingCore() with no regex constraint registered, plus eight route cases: .json, .yaml, .yml, .JSON and .YAML serve 200, and .txt, .html and .jsonx fall through to a 404. All nine fail against master's source with the same RouteCreationException from the report. With the change, Swashbuckle.AspNetCore.SwaggerGen.Test is 769 passed, 0 failed on net8.0. I also confirmed the standalone slim-builder app end to end: it crashed before, and after the change it starts, serves a valid document at swagger.json, serves swagger.yaml as text/yaml, and 404s swagger.txt.

Two things I did not change, deliberately. .YAML returns 200 with JSON content, because the old regex was case-insensitive while the json/yaml switch downstream is case-sensitive; that is master's existing behaviour and this PR is not the place to change it. And a user with a custom, unconstrained {extension} template, say docs/{documentName}/spec.{extension}, previously got JSON for any extension and now sees anything outside the allow-list fall through to the next middleware. That is the same tightening #2418 intended for the default template, but it is a behaviour change and worth your call.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.32%. Comparing base (d0b1e26) to head (1af6de0).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4130      +/-   ##
==========================================
+ Coverage   95.24%   95.32%   +0.08%     
==========================================
  Files         111      111              
  Lines        4142     4149       +7     
  Branches      848      850       +2     
==========================================
+ Hits         3945     3955      +10     
+ Misses        197      194       -3     

Flag	Coverage Δ
Linux	95.32% <100.00%> (+0.08%)	⬆️
Windows	95.32% <100.00%> (+0.08%)	⬆️
macOS	95.32% <100.00%> (+0.08%)	⬆️

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[RaphaelFakhri]

Both suggestions are applied: the test is renamed to UseSwagger_Works_With_AddRoutingCore, and the explanatory comment in it is removed, since the new name says what the test covers.

On the PublicAPI.Unshipped.txt note: agreed, only the default value of the pattern parameter changes. The extension check that the regex performed now runs in the middleware for the default template, and custom templates that specify their own constraints behave as before.

[github-actions]

Thanks for your contribution @RaphaelFakhri - the changes from this pull request have been published as part of version 10.3.0 📦, which is now available from NuGet.org 🚀