feat(investigations): add the investigation template registry [3/13]

[arslnb]

Add the code-owned, versioned template registry used by Investigation creation, plus the first breached-metric template. Templates are declarative specs with no database or service logic; instantiation comes in the next PR.

[sentry]

Bug: The frozen=True dataclasses TemplateBlockSpec and TemplateParameterSpec are unhashable because they contain dict fields, which will cause a TypeError if they are ever used in sets or as dict keys.
_{Severity: LOW}

Suggested Fix

To make the dataclasses hashable as intended by frozen=True, convert the dict fields to an immutable type. For example, change dict[str, Any] to MappingProxyType[str, Any] and ensure the dictionary is wrapped in MappingProxyType during initialization. This preserves the immutable intent while making the objects correctly hashable.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: src/sentry/investigations/templates/types.py#L18-L30

Potential issue: The dataclasses `TemplateBlockSpec` and `TemplateParameterSpec` are
defined with `frozen=True`, which implicitly generates a `__hash__` method. However,
these classes contain fields of type `dict` (`config`, `display`, `constraints`), which
are unhashable. Any attempt to add instances of these classes to a set or use them as
dictionary keys will result in a `TypeError: unhashable type: 'dict'`. While the current
pull request does not perform any hashing operations on these objects, this design
creates a latent bug that will likely cause runtime crashes in future code that handles
instantiation, caching, or deduplication of these spec objects.

Also affects:

• src/sentry/investigations/templates/types.py:7~16

[arslnb]

LGTM!