[FIX]: Enforce PromptTemplate construction invariants

[spencrr]

Description

Fixes construction and equality invariants for PromptTemplate introduced in #120.

The dataclass-generated initializer exposed every field, including _template. A leading underscore is only a Python naming convention, so a caller could combine declared parameter metadata with an unrelated Jinja template and bypass PromptTemplate.from_yaml() validation:

PromptTemplate(
    name="x",
    description=None,
    parameter_keys=("declared",),
    _template=Template("{{ actual }}"),
).render(declared="provided")

This renders an empty string: argument validation succeeds against parameter_keys, while the injected template references a different variable and was compiled outside RAMPART's StrictUndefined environment.

This PR:

• disables the generated dataclass initializer and explicitly rejects direct construction, including the otherwise-valid empty PromptTemplate() call left available by init=False alone;
• creates instances only from a schema-validated YAML definition;
• compiles the Jinja template inside the validated-definition factory so the declared parameters and compiled behavior cannot drift independently;
• uses object.__new__() and object.__setattr__() for the narrow initialization phase required by a frozen, slotted dataclass;
• disables generated structural equality, which previously treated templates with identical metadata but different bodies as equal because _template was excluded from comparison, while preserving identity-based hashing; and
• marks PromptTemplate as final for static type checkers and IDEs.

PromptTemplate.from_yaml(path) remains the supported construction API and retains its Self return type. Separate instances now use identity equality:

first = PromptTemplate.from_yaml(path)
second = PromptTemplate.from_yaml(path)

assert first is not second
assert first != second
assert len({first, second}) == 2

Breaking changes

Direct PromptTemplate(...) construction is now rejected. Use PromptTemplate.from_yaml(path) instead. Equality between separate instances is now identity-based rather than metadata-based. Existing RAMPART production call sites already construct templates through from_yaml().

Checklist

• pre-commit run --all-files passes
• Tests added or updated for changes: direct construction, the former generated-constructor arguments, identity equality, and hashability
• Documentation updated: factory-only construction and migration are documented above and in the API docstrings

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