Closes #714

• Multipart/form-data is not rerouted (error 404) #714

Summary

Adds regression coverage and a real browser sample for #714 against the current develop branch.

The original issue reported that multipart/form-data requests containing a file returned 404 instead of being rerouted. After validating the current request pipeline, the original issue is no longer reproducible for regular, non-aggregated routes: Ocelot streams the raw request body and preserves the original Content-Type, including the multipart boundary.

No production-code change under src/ is required for the regular rerouting scenario. This PR documents and protects that behavior.

Coverage added

• Acceptance coverage that reroutes POST /upload to /api/files using multipart/form-data
• Verifies that the downstream route is reached successfully
• Verifies the downstream path
• Verifies the multipart Content-Type and boundary
• Verifies a regular form field
• Verifies the file field name, original filename, file content type, and exact file contents
• Unit coverage for RequestMapper confirming that the multipart body and Content-Type boundary are preserved unchanged
• Unit coverage for the multiplexer context clone confirming that a cloned multipart request body can still be read as form data without copying Request.Form

Real sample

Adds samples/MultipartFormData with two small apps:

• ApiGateway: serves a valid HTML page containing a real enctype="multipart/form-data" form with a text field and file input, then forwards POST /upload through Ocelot
• DownstreamService: handles POST /api/files, reads the request using ReadFormAsync, and returns the received form field, file metadata, file contents, and request content type

The sample README includes the run instructions and uses only the existing ASP.NET Core/Ocelot project setup.

Review follow-up

• The one-test constants in the acceptance test were moved to local variables and passed into helpers where needed.
• The requested real sample/browser form scenario was added with an actual HTML upload page.
• I reviewed the Form = from.Form idea from Add 'multipart/form-data' support for file uploads #1372. It was not technically necessary for this PR: the regular route works through the existing raw-body streaming path, and the multiplexer clone can read multipart form data from the cloned body plus copied content headers. Therefore this PR intentionally does not add Form = from.Form.
• POST aggregation/multiplexing support remains out of scope and should be treated as a future feature/design discussion.

Validation

Validated locally with .NET SDK 10.0.400:

• dotnet restore Ocelot.slnx passed
• dotnet restore Ocelot.Samples.slnx passed, with existing ServiceFabric/Steeltoe dependency warnings outside this PR
• dotnet build Ocelot.slnx --no-restore passed with 0 warnings/errors
• dotnet build Ocelot.Samples.slnx --no-restore passed, with the same existing ServiceFabric/Steeltoe warnings
• dotnet format Ocelot.slnx --no-restore --verify-no-changes --include acceptance/Request/MultipartFormDataTests.cs unit/Multiplexing/MultiplexingMiddlewareTests.cs unit/Request/Mapper/RequestMapperTests.cs passed
• dotnet format Ocelot.Samples.slnx --no-restore --verify-no-changes --include samples/MultipartFormData/ApiGateway/Program.cs samples/MultipartFormData/DownstreamService/Program.cs passed
• Issue Multipart/form-data is not rerouted (error 404) #714 unit tests passed on net8.0 and net10.0
• Issue Multipart/form-data is not rerouted (error 404) #714 acceptance test passed on net8.0 and net10.0
• Full unit suite passed on net8.0 and net10.0 with --culture invariant (1901 passed per TFM)
• Full acceptance suite on net10.0 was run; it had one unrelated ClientWebSocketTests.Http20ClientWhenDirectConnectionThenShouldConnect HTTP/2 WebSocket timeout. Rerunning the suite excluding only that test passed (358 passed, 3 skipped).
• The new sample was run manually: GET http://localhost:5567/ returned 200, and a real multipart POST /upload through the gateway returned 200 with the expected text field, file metadata, exact file contents, and multipart boundary received downstream.

Local note: net9.0 builds successfully, but the .NET 9 runtime is not installed in this environment, so local test execution was limited to net8.0 and net10.0.

Production impact

No production files under src/ are changed.

The current implementation already supports the regular multipart/form-data rerouting scenario, so this PR adds regression coverage and a real executable sample to prevent that behavior from being unintentionally broken in the future.

Refs #714