build(mypy): ignore missing stubs for sentence_transformers

[RobertSigmundsson]

Problem

sentence_transformers ships no type stubs. It is absent from the existing
[[tool.mypy.overrides]] block that already lists networkx.*, chromadb.*, openpyxl.*,
google.genai.* and watchdog.* — so in any environment where the package is installed, mypy
reports import errors for it while every comparable dependency is exempt.

Change

One line in pyproject.toml:

[[tool.mypy.overrides]]
module = [
    ...
    "watchdog.*",
+   "sentence_transformers.*",
]
ignore_missing_imports = true

Why upstream benefits

Consistency with every other stub-less optional dependency already handled the same way, and a clean
mypy run for contributors who have the package installed.

Evidence

Check	Result
cherry-pick onto origin/main	clean
import surreal_memory	OK
ruff check	All checks passed!
ruff format --check	701 files already formatted
mypy src/ --ignore-missing-imports	Success: no issues found in 369 source files
pytest -m "not stress" -n auto	6535 passed, 84 skipped, 1 xfailed — identical to baseline

Risks / notes for the reviewer

• Effectively none. The gate passes with and without it in an environment where the package is not
  installed; the change only matters where it is.
• Worth merging first — it is a one-liner and it is a prerequisite for a clean mypy on any branch that
  imports sentence_transformers.