Migrate to Bazel 9

[yukawa]

Description

Like we did in the last year to switch from Bazel 7.x to Bazel 8.x, we need some extra work to fully migrate to Bazel 9.

• Migrate to Bazel 8.0 #1118

Remaining tasks are:

• Make sure all the dependent Bazel modules are compatible with Bazel 9.x
  • Update module versions as needed.
• Make sure Mozc's own build rules are compatible with Bazel 9.x
  • Update Mozc's own build rules as needed.
• Once Mozc becomes fully compatible with Bazel 9.x, update USE_BAZEL_VERSION in src/.bazeliskrc
• (optional) Evaluate the implications of prebuilt proto compiler:
  • https://protobuf.dev/news/2026-01-16/

See also the following discussion:

• Build error: bazel 9 support #1436

Version or commit-id

76cb1ce

Additional Context

Right now, the Mozc team is verifying that the Windows and macOS binaries built with Bazel are production-ready, which is the final step before we can deprecate GYP. Since our primary focus is replacing GYP, we want to avoid introducing build tool upgrades that might destabilize the environment during this critical verification phase. Once the GYP deprecation is finalized, we will be in a better position to update dependencies like Bazel.

Thus pull requests that try to update bzlmod modules are unlikely to be merged until we announce that GYP build is fully deprecated.