chore: add s2-specs as a submodule

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[greptile-apps]

Greptile Summary

This PR converts the previously-tracked s2-specs files into a proper git submodule and introduces an update_specs script to regenerate protobuf bindings from it. The runtime protobuf dependency is correctly bumped to >=6.31.1,<7.0.0 to match the newly generated code, and the generated s2_pb2.py/s2_pb2.pyi files are updated accordingly.

Confidence Score: 5/5

Safe to merge; all substantive dependency and generated-code issues are resolved in this PR

No P0 or P1 findings in the current diff. The previously-flagged P1 (protobuf version mismatch) has been addressed. Only a minor P2 style suggestion on update_specs and the pre-existing types-protobuf stubs version remain.

No files require special attention; pyproject.toml types-protobuf stubs version mismatch is minor and already noted in previous review rounds.

Important Files Changed

Filename	Overview
pyproject.toml	Runtime protobuf dependency correctly bumped to >=6.31.1,<7.0.0; dev types-protobuf still pinned at a 5.x stub version
update_specs	New script to regenerate protobuf bindings from submodule; uses sed -i '' (macOS-only, already discussed) and unquoted xargs pipeline
s2-specs	New git submodule pointing to s2-streamstore/s2-specs at commit a47ed3e; .gitmodules correctly configured
src/s2_sdk/_generated/s2/v1/s2_pb2.py	Regenerated with protobuf 6.31.1; ValidateProtobufRuntimeVersion call now requires protobuf 6.x, consistent with the dependency bump
src/s2_sdk/_generated/s2/v1/s2_pb2.pyi	Updated generated type stubs matching the regenerated s2_pb2.py

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[s2-specs submodule\na47ed3e] -->|s2/v1/s2.proto| B[update_specs script]
    B -->|grpc_tools.protoc| C[s2_pb2.py\ns2_pb2.pyi]
    B -->|sed import fix| C
    B -->|poe cq-fix| D[Linted & formatted generated code]
    C --> D
    D --> E[src/s2_sdk/_generated/s2/v1/]

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Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
update_specs:13
**`find | xargs` pipeline lacks null-delimiter safety**

If any `.py` file path contains spaces or special characters, the plain pipe to `xargs -I{}` will split the path incorrectly, causing `sed` to receive a broken filename. Use `-print0` with `xargs -0` to handle arbitrary filenames safely.

```suggestion
find . -name '*.py' -print0 | xargs -0 -I{} sed -i '' 's/from s2\.\(v[0-9][a-z0-9]*\) import s2_pb2/from s2_sdk._generated.s2.\1 import s2_pb2/' {}
```

_{Reviews (2): Last reviewed commit: "fix proto dep" | Re-trigger Greptile}

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@greptileai review again