Fix crash when after_unknown is an object in output changes

This patch fixes a DeserializeUnableToConvertValue exception that occurred when
parsing Terraform plan JSON where output_changes[*].after_unknown is a non-boolean
value (for example {}).

🐛 Bug fixes

Fixed parser crash for output changes with non-boolean after_unknown

Problem: When a Terraform plan contains an output change whose after_unknown field
is an object ({}) instead of a plain boolean (true / false), tfplan2md crashed
with a deserialization exception:

System.Text.Json.JsonException: DeserializeUnableToConvertValue

This can happen with certain provider output types — for example, when an entire
computed output value is unknown at plan time and Terraform encodes that state as an
object rather than true.

Root cause: OutputChange.AfterUnknown was typed as bool in the internal plan
model. The Change record — used for resource attribute changes — was already typed
as object? and handled via AfterUnknownHelper.IsWholeResourceUnknownAfterApply(),
but OutputChange was missed when output-change support was added.

Fix: Changed OutputChange.AfterUnknown from bool to object?, matching the
existing Change pattern. The consumer (ReportModelBuilder.Outputs.cs) now uses
AfterUnknownHelper.IsWholeResourceUnknownAfterApply() to derive the IsComputed
flag, so behaviour for true / false values is unchanged and {} is correctly
treated as "not entirely unknown".

Impact:

• Plans that previously caused a crash now parse and render correctly.
• Output changes with after_unknown: true still render with a "(computed)" indicator.
• Output changes with after_unknown: {} or after_unknown: false render normally,
  with no crash and no change to the displayed output value.

🔗 Commits

• 1ca12c9 fix: change OutputChange.AfterUnknown from bool to object?