Update Stone submodule

[aaronsky]

Summary

• Bump the Stone submodule to dropbox/stone main at 7a087d2d592bcc4500511b15109eeffa6ba29259.
• Stop forwarding SwiftyDropbox's documentation flag to Stone, since the new Stone output-root validation rejects the backend's historical out-of-root jazzy output path.
• Leave generated SDK sources unchanged after formatting; this is intended as a minimal compatibility update.

Test Plan

• uv run --no-project --with-requirements stone/requirements.txt ./generate_base_client.py
• uv run --no-project --with-requirements stone/requirements.txt ./generate_base_client.py --objc
• uv run --no-project python -m py_compile generate_base_client.py
• git diff --check
• find Source/SwiftyDropbox Source/SwiftyDropboxObjC -name '*.swift' -exec swiftc -parse -parse-as-library {} +
• CocoaPods/Xcode integration tests were not run locally.