Implement support for #133 ([AsParameters]) + help text fixes

[dusrdev]

Summary

• Added first-class support for grouping command parameters into a single record via [AsParameters].
• Expanded grouped constructor parameters into normal CLI-bound parameters during source generation, then re-hydrated the record at invocation time.
• Included two help text quality fixes discovered during implementation:
• Fixed option suffix spacing ([Required] / [Default: ...]) when option description is empty.
• Fixed XML <summary> multi-line normalization so multi-line command summaries render correctly in help output.

New capability: [AsParameters]

• Introduced [AsParameters] attribute support on command parameters.
• Supported usage across ConsoleApp.Run, method references, ConsoleAppBuilder.Add(...), and class command registration.
• Implemented generator pipeline support to distinguish:
• Runtime parameters (actual delegate/method signature).
• Effective parse parameters (flattened parameter list used for CLI parsing/help/validation).
• Added explicit expansion mapping metadata (AsParametersBinding) to map runtime parameter index to flattened constructor parameter indexes.
• Emission now:
• Parses flattened parameters exactly like regular parameters.
• Performs validation against the correct ParameterInfo source (method parameter vs expanded constructor parameter).
• Constructs the target record (new TargetType(...)) and passes it into the original command signature.

Supported behavior for expanded constructor parameters

• Normal option binding semantics (name conversion, required/default behavior).
• [Argument] ordering support.
• [FromServices] and [FromKeyedServices] resolution.
• DataAnnotations validation attributes.
• [Hidden] and [HideDefaultValue] help behavior.
• Custom parser attributes implementing IArgumentParser<T>.
• XML <param> aliases/descriptions applied from constructor parameter docs.
• Mixing with regular command parameters, CancellationToken, ConsoleAppContext, multiple [AsParameters] parameters, and global options.

Guardrails and diagnostics

• Added diagnostics specific to [AsParameters]:
• CAF019: target must be a record class.
• CAF020: target must have exactly one public instance constructor.
• CAF021: nested [AsParameters] is not supported on constructor parameters.
• CAF022: params constructor parameters are not supported.
• Added duplicate option/alias detection across the effective flattened parameter set:
• CAF023: duplicate option name or alias.
• Extended existing doc-comment parameter-name mismatch validation (CAF015) to constructor XML params used by [AsParameters].

Help text fixes (added during this work)

• Option suffix spacing fix:
• CommandHelpBuilder now appends a separating space before [Required] / [Default: ...] only when description text is non-empty.
• This prevents extra spacing artifacts and keeps help formatting consistent for blank-description options.
• Multi-line summary rendering fix:
• RoslynExtensions.GetSummary() now normalizes summary text line-by-line.
• Trims leading/trailing blank lines and preserves intentional line breaks in multi-line <summary> blocks.
• Produces cleaner, stable help output for multi-line command descriptions.

Tests

• Added comprehensive AsParametersTest coverage including:
• Basic flatten/invoke.
• Defaults and nullable defaults.
• Validation behavior.
• Services and keyed services.
• Help parity against equivalent expanded signatures.
• Method references, builder registration, class registration.
• Interop with argument ordering, cancellation token, context, global options, custom parsers, hidden/default-hidden settings, and multiple [AsParameters] instances.
• Added diagnostics tests for CAF019-CAF023 and constructor doc-comment mismatch.
• Added help regression test for multi-line summary output.
• Updated expected help snapshots for spacing changes from the option suffix formatting fix.

Documentation

• Added new README section: AsParameters.
• Included usage example, XML alias/description example, and current constraints.

Notes for reviewers

• This change does not introduce a separate AsParameters-specific parser/emitter/help pipeline.
• The implementation reuses the existing parsing/help/validation/generation flow by introducing EffectiveParseParameters (a flattened view of parameters).
• Runtime signature shape is preserved via Parameters; only the final invocation path adds a rehydration step (new TargetType(...)) before calling the original method/delegate.
• The key internal refactor is therefore a data-model split (runtime vs effective parse parameters), not a parallel processing stack.

Closes #133

[neuecc]

Thank you.
I want to prioritize consistency in naming (and behavior), so I think this is preferable to #237.
Please give me a little time to review the code.