Replace IResultMapper with ResultMapper base class

[bfarmer67]

Description

• Replace IResultMapper interface with ResultMapper base class with overridable virtual methods for each decision point:

  • MapException
  • GetStatusCode
  • MapValidationFailures
  • MapCancellation
  • MapSuccess

• Add ResultMapper.Create() factory for one-off lambda customizations without subclassing

• Add chainable IResult decorators: .WithHeader() and .WithNoContent()

• Change default validation status code from 400 to 422 (RFC 9110)

• Change default exception handling from throw to ProblemDetails 500

• Null success results now return 404 NotFound instead of Ok(null)

• Remove where TResult : class constraint from contentSelector overload

• Rename selector parameter to contentSelector

Motivation

User code review identified three gaps in the result mapping API:

• No way to decorate successful responses (e.g., ETag headers) without bypassing error handling
• Hardcoded status codes in TryHandleInvalidCommand with no extension point
• IResultMapper could not compose with selector overloads - no overload accepted both a mapper and a contentSelector

The user built 80 lines of custom extension methods as workarounds. With this PR, all three patterns reduce to:

// Single GET with ETag
return commandResult.ToResult( x => x?.ToPresentation(), conflictMapper )
    .WithHeader( "ETag", commandResult.Result?.VersionTag );

// Collection GET
return commandResult.ToResult( x => x.ToPresentationList() );

// Write with conflict handling
return commandResult.ToResult( conflictMapper ).WithNoContent();

Breaking changes

• IResultMapper interface removed (released 2 days ago)
• Default validation status code: 400 -> 422
• Default exception handling: throws CommandException -> returns ProblemDetails 500 (override MapException returning null to restore throw behavior)
• Null success result: Ok(null) -> NotFound

Tests

• 68 AspNetCore tests covering all ResultMapper virtual methods, decorator chaining, cancellation, priority status codes, lambda factory, and all ToResult overloads
• 228 total tests pass across net8.0, net9.0, net10.0