Handle oversized event post bodies

[ejsmith]

Summary

• add a streaming request-body guard for event POSTs so unknown-length bodies can be rejected after the app limit is reached
• keep Kestrel slightly above the app event-post limit so the controller can return the client-facing status first
• prevent rejected/truncated event post payloads from being queued and clean up partial saved files

Behavior

• unknown-length event POSTs over MaximumEventPostSize now return 413 Payload Too Large
• expected request body read failures are converted to their HTTP status instead of surfacing as unhandled exceptions
• oversized rejected posts increment the existing too-big usage counter

Verification

• dotnet test -- --filter-class Exceptionless.Tests.Utility.EventPostRequestBodyStreamTests
• dotnet test -- --filter-class Exceptionless.Tests.Services.EventPostServiceTests
• dotnet test -- --filter-class Exceptionless.Tests.Controllers.EventControllerTests
• dotnet test -- --filter-class Exceptionless.Tests.Controllers.ControllerManifestTests
• dotnet test -- --filter-class Exceptionless.Tests.Controllers.OpenApiControllerTests

Breaking Changes

• None

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
Exceptionless.AppHost	39%	40%	134	❌
Exceptionless.Core	71%	63%	8469	➖
Exceptionless.Insulation	24%	23%	203	❌
Exceptionless.Web	75%	63%	4773	➖
Summary	70% (15961 / 22655)	62% (8123 / 13076)	13579	➖