feat(gen-shacl): add --message-template for sh:message on property shapes

[jdsika]

Summary

Adds a --message-template CLI option to gen-shacl that generates sh:message literals on every SHACL property shape from a user-supplied template string. This enables SHACL validators (e.g. pyshacl) to produce human-friendly error messages identifying the specific constraint that was violated.

Depends on: #9 (--default-language flag)

Problem

LinkML's SHACL generator currently produces property shapes without sh:message. When SHACL validation fails, the error report contains only technical details (focus node, result path, constraint component) but no human-readable explanation. Users must manually trace the violation back to the source shape.

The SHACL specification (§2.1.5) explicitly supports sh:message for this purpose:

"Values of sh:message are either xsd:string literals or literals with a language tag."

Usage

gen-shacl --message-template '{class}.{name}: {title}' schema.yaml
gen-shacl --message-template 'Validation of {name} failed!' --default-language en schema.yaml

Supported placeholders

Placeholder	Description	Example
{name}	Slot name (underscore-separated LinkML identifier)	has_speed
{title}	Slot title (human-readable), falls back to name	Has Speed
{class}	Enclosing class name	Vehicle
{path}	Property IRI	http://example.org/hasSpeed

Changes

gen-shacl (shaclgen.py)

• New message_template field with __post_init__ whitespace normalisation
• Template expansion after sh:name / sh:description emission
• Uses prop_pv_text for language tag support when combined with --default-language (sh:message gets language tag per SHACL §2.1.5)
• Invalid/positional placeholders and format specs raise ValueError with helpful message (catches KeyError, IndexError, ValueError)
• Empty and whitespace-only templates normalised to None (no sh:message emitted)
• New --message-template Click CLI option

Tests (10 new)

• Basic template with {name}, {title}, {class}, {path}
• Title/class placeholder expansion
• Backward compatibility (no template = no sh:message)
• Invalid placeholder raises ValueError
• Positional placeholder {0} raises ValueError
• Format spec {name:d} raises ValueError
• Empty string normalised to no emission
• Whitespace-only normalised to no emission
• Combined with --default-language (sh:message gets @en tag)

Backward compatibility

• Default is None — existing behaviour is completely unchanged
• Empty strings and whitespace-only values normalised to None

Standards compliance

Standard	Requirement	Status
SHACL §2.1.5	sh:message accepts xsd:string or rdf:langString	✅
SHACL Core	sh:message is non-validating metadata for error reports	✅
pyshacl	Preserves @lang tag on sh:resultMessage in reports	✅ Verified

Testing

• 39 SHACL tests pass (22 existing + 7 C1 + 10 C2), 0 skipped
• 3 rounds of adversarial review completed with 0 open bugs