feat: add generic typing support for Variable.get method

[devin-ai-integration]

closes #19752

This PR adds optional explicit typing for the Variable.get and Variable.aget methods, allowing users to declare the expected type when retrieving a variable:

var = Variable[str].get("my_variable")

Implementation

• Makes Variable class generic with TypeVar T bounded to StrictVariableValue
• Uses TypeVar with default=StrictVariableValue from typing_extensions for backward compatibility (unparameterized Variable.get() still works as before)
• Adds overloads for get/aget to properly type the return value:
  • Without default: returns T | None
  • With default: returns T
• Both sync (get) and async (aget) methods support the same typing behavior

Human Review Checklist

• Verify the overload signatures correctly capture the intended typing behavior
• Confirm backward compatibility - existing code using Variable.get() without type parameter should continue to work
• The return type annotation changed from StrictVariableValue to StrictVariableValue | None which is more accurate - verify this doesn't cause issues elsewhere

Checklist

• This pull request references any related issue by including "closes <link to issue>"
• If this pull request adds new functionality, it includes unit tests that cover the changes
• If this pull request removes docs files, it includes redirect settings in mint.json.
• If this pull request adds functions or classes, it includes helpful docstrings.

---

Link to Devin run: https://app.devin.ai/sessions/d315656d34b445ee9e1d1ca820411d24
Requested by: Nate Nowack ([email protected]) (@zzstoatzz)

[devin-ai-integration]

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[codspeed-hq]

CodSpeed Performance Report

Merging #19764 will not alter performance

_{Comparing devin/1765568895-variable-generic-typing (a95c98a) with main (57d6b87)}

Summary

✅ 2 untouched

[zzstoatzz]

checked it out locally and LGTM, but want a sanity check from @desertaxle !

[desertaxle]

LGTM!