Improve dictionary construction errors

[tuckermcclure]

Summary

This PR improves the error reported when from_dict cannot construct a requested type from a dictionary.

Previously, the final error printed the complete dictionary and always suggested adding a type key. That suggestion could be misleading when a type tag had already resolved to a concrete type and construction subsequently failed because its fields did not match.

The new error explains the construction strategies PortableStructs considered and identifies the relevant key mismatch.

Changes

• Report the requested type and supplied dictionary keys.
• Explain when no keyword constructor accepts the supplied keys.
• Report expected, missing, and unexpected fields for positional construction.
• Avoid printing the complete dictionary, which may be large or contain sensitive values.
• Suggest identifying a concrete subtype only when the requested type is Any or abstract.
• Safely handle dictionary keys whose conversion to Symbol fails.
• Use the same key analysis for keyword-constructor detection, positional matching, and error reporting.

This does not add keywords or change the from_dict extension interface. Construction failures remain ErrorExceptions.

Examples

For a concrete type with mismatched keys:

Could not construct a ConcreteConfig from a dictionary.

Supplied keys: ("name", "extra")
No keyword constructor accepts the supplied keys.
Positional construction requires the supplied keys to match the fields exactly.
Expected fields: (:name, :count)
Missing fields: (:count,)
Unexpected keys: ("extra",)

If a type tag resolves to ConcreteConfig but its dictionary contains the wrong fields, the error describes that concrete construction failure:

Could not construct a ConcreteConfig from a dictionary.

Supplied keys: ("wrong",)
No keyword constructor accepts the supplied keys.
Positional construction requires the supplied keys to match the fields exactly.
Expected fields: (:name, :count)
Missing fields: (:name, :count)
Unexpected keys: ("wrong",)

It does not incorrectly suggest adding another type tag.

For an untagged abstract type, the error retains a useful subtype suggestion:

Could not construct a VehicleConfig from a dictionary.

Supplied keys: ("name",)
No keyword constructor accepts the supplied keys.
Positional construction requires the supplied keys to match the fields exactly.
Expected fields: ()
Unexpected keys: ("name",)
If this dictionary represents a concrete subtype, identify it with a "type" key.

Testing

Added coverage for:

• Missing and unexpected fields on concrete types.
• Tagged concrete types with mismatched fields.
• Untagged abstract types.
• Keys whose conversion to Symbol throws.
• Preservation of the existing ErrorException type.

The complete PortableStructs test suite passes:

julia --project=test -e 'import Pkg; Pkg.test("PortableStructs")'