Declare the output contract for Json's public commands

[MariusStorhaug]

Context and request

None of the three public commands declares [OutputType()], and none documents .INPUTS, .OUTPUTS, or .NOTES. The MSX PowerShell function standard treats [OutputType()] and its matching .OUTPUTS block as the contract between a function and its callers, and PlatyPS renders both into the generated reference pages published at psmodule.io/Json. Today those pages carry no output contract at all.

Each command already has .SYNOPSIS, .DESCRIPTION, at least one .EXAMPLE, and a correct .LINK, so this completes existing help rather than writing it from scratch.

Acceptance criteria.

• Format-Json, Import-Json, and Export-Json each declare [OutputType()], scoped per parameter set where the sets return different types.
• Each documents .INPUTS and .OUTPUTS, with a description for every entry and fully-qualified .NET type names.
• .OUTPUTS matches [OutputType()] exactly.
• Where no parameter accepts pipeline input, .INPUTS reads None with You can't pipe objects to <CommandName>. as its description.
• Get-Help <Command> -Full shows inputs and outputs for all three commands.
• The generated reference pages render the type as a heading with the description as body text below it.

Technical decisions

.INPUTS and .OUTPUTS use the PlatyPS blank-line format — type name on the first line, blank line, then the description as a plain paragraph:

.OUTPUTS
System.String

The formatted JSON text.

The single-line System.String. Description. form shown in about_Comment_Based_Help is not used: it triggers MD026 and puts a full sentence inside a ### heading when PlatyPS v2 processes it.

Type names are fully qualified — System.String, System.Management.Automation.PSCustomObject, System.IO.FileInfo — not accelerators.

Export-Json needs care on [OutputType()]. It emits System.IO.FileInfo only when -PassThru is supplied and nothing otherwise. Since that is a switch rather than a parameter set, a single unscoped [OutputType([System.IO.FileInfo])] is correct, with .OUTPUTS stating that output is produced only with -PassThru.

Import-Json currently emits objects carrying an injected _SourceFile property. The .OUTPUTS description documents the corrected contract — the deserialized document — not the current polluted one, so this should land after or alongside that correction.

This is a documentation and attribute change with no runtime behaviour change, which is why it is separated from the structural work on the same functions. src/ matches ImportantFilePatterns, so it still triggers a full build and warrants a Patch label.

Implementation plan

• Add [OutputType([System.String])] to Format-Json with .INPUTS covering both pipeline-bound parameter sets and .OUTPUTS describing the formatted JSON text
• Add [OutputType([System.Management.Automation.PSCustomObject])] to Import-Json with .INPUTS for the pipeline-bound Path and .OUTPUTS describing the deserialized document
• Add [OutputType([System.IO.FileInfo])] to Export-Json with .INPUTS covering both pipeline-bound sets and .OUTPUTS noting output only occurs with -PassThru
• Add .NOTES where there is genuinely something a caller needs to know, and omit it where there is not
• Verify Get-Help <Command> -Full renders all sections correctly for each command
• Confirm the generated documentation pages render the heading and body separation as intended
• Confirm .OUTPUTS and [OutputType()] agree for every command