Smidge 5: Nuglify-only, .NET 10, minimal APIs, and graceful 404 fixes - #223

Smidge 5 drops its dependency on MVC for serving bundles. The two controllers
and their action filters are replaced with minimal API endpoints and endpoint
filters, so AddSmidge no longer forces MVC startup on the host. Tag helpers stay
in the Smidge package (they keep the only Razor dependency).

- Request models now use IHttpContextAccessor + Request.RouteValues instead of
  the obsolete IActionContextAccessor
- The 4 action filters become IEndpointFilters (compression, expiry,
  not-modified, cache short-circuit), added outer-to-inner in the same order the
  MVC filter Order produced so behavior is preserved
- SmidgeController -> SmidgeRequestHandler and NuglifySourceMapController ->
  NuglifySourceMapHandler: POCO handlers returning IResult
- SmidgeStartup: drop AddMvcCore/AddApplicationPart and the IActionContextAccessor
  registration; register the handlers; UseSmidge maps three MapGet endpoints with
  the endpoint-filter chain
- Remove the legacy useEndpointRouting/UseMvc branch and parameter (breaking)
- Delete the unused BundleModelBinder

Verified against the sample app: bundle/composite endpoints return 200 with the
correct caching headers, If-None-Match yields 304, and tag helpers still render
bundle URLs.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Requests for composite files and Nuglify source maps could throw an
unhandled FileNotFoundException that surfaced as a 500. Because the
composite URL and source-map requests contain client-supplied values
(and browsers request source maps lazily), this was easy to trigger
repeatedly - a denial-of-service vector reported in #199 and the 500
seen for the notfound-map scenario in #185.

Adds a non-throwing ICacheFileSystem.GetFileInfo(string) alongside the
existing GetRequiredFileInfo (which stays throwing for genuine internal
invariants). The composite and source-map request handlers now use the
non-throwing lookup and return a graceful 404 (with a log entry) when a
requested file is missing or stale.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

These types are implementation details invoked from the minimal API
endpoints wired up in UseSmidge; they were never intended to be part of
the public API surface. Marking them internal avoids committing to
supporting them as public APIs in Smidge 5.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Covers the new non-throwing GetFileInfo alongside the throwing
GetRequiredFileInfo for both the in-memory and physical cache file
systems, locking in the behaviour the graceful 404 fix relies on.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Adds a Smidge.Integration.Tests project that self-hosts Smidge on Kestrel
and exercises the same scenarios covered manually by the Smidge.Web
sample views: production and debug bundles, dynamic composite files,
source maps (served and gracefully 404'd), spoofed composite requests,
empty bundles, conditional (304) requests and gzip compression.

The whole suite runs twice via IClassFixture, once against the in-memory
cache and once against the physical cache, to guard both code paths
including the graceful 404 handling for missing cached/source-map files.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Migrates Smidge.sln to the XML-based Smidge.slnx solution format and
updates the CI build workflow to reference it.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

- RequestModel: throw a clear InvalidOperationException when no active
  HttpContext is available instead of a NullReferenceException, and give
  the valueName ArgumentException a meaningful message.
- CI: bump actions/setup-dotnet and actions/checkout to v4.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Per RFC 7232 a request that contains an If-None-Match header must ignore
If-Modified-Since. The previous OR-based check could return 304 when a
non-matching ETag was combined with an If-Modified-Since indicating the
content was unchanged. The filter now evaluates ETag precedence first and
only falls back to the modified-since date when no If-None-Match header is
present. Adds an integration regression test covering the mismatched-ETag
plus unmodified-since case.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Enables TreatWarningsAsErrors solution-wide via a repo-root
Directory.Build.props (chained from src/Directory.Build.props so the
src projects pick it up too) and resolves the outstanding build
warnings:

- Smidge.Web: replace the obsolete WebHost/IWebHost startup (ASPDEPR008)
  with the generic Host.CreateDefaultBuilder().ConfigureWebHostDefaults()
  pattern returning IHost.
- Smidge.Tests: drop the redundant System.Diagnostics.TraceSource
  package reference flagged by NU1510.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>