Upgrade to Bazel 7.7.0, rules_python 1.0.0, and protobuf 29.5

[rjhuijsman]

Summary

This PR upgrades the Bazel build to Bazel 7.7.0 as part of a coordinated
upgrade across the entire monorepo. It also upgrades rules_python from
0.27.0 to 1.0.0 and protobuf from an older version to 29.5.

Why things had to change

Bazel 7.7.0 (.bazeliskrc)

Bumped USE_BAZEL_VERSION from 5.4.1 to 7.7.0.

bzlmod disabled (.bazelrc)

Bazel 7 enables bzlmod by default. Added common --enable_bzlmod=false
to keep the existing WORKSPACE-based build working until the project
is ready to migrate to MODULE.bazel.

repo_mapping parameter removed (bazel/repos.bzl, bazel/deps.bzl, bazel/py_toolchains.bzl)

Bazel 7 removed the repo_mapping parameter from repository rules
(http_archive, git_repository, etc.). All explicit repo_mapping = {}
arguments have been removed.

native.existing_rules() deprecated; external parameter removed (bazel/repos.bzl, WORKSPACE.bazel)

Bazel 7 deprecates native.existing_rules(). The pattern of accepting
an external parameter to conditionally register the repo's own
git_repository (so downstream consumers could pull it in) is replaced
throughout with maybe() from @bazel_tools//tools/build_defs/repo:utils.bzl,
which already skips registration if the repo already exists.

rules_python 1.0.0 (bazel/repos.bzl, bazel/pypi_repos.bzl)

Upgraded from 0.27.0 to 1.0.0, required by protobuf 29.x which loads
rules_python internally. In 1.0.0, the @python3_10_12//:defs.bzl
interpreter symbol no longer exists; it is replaced by the label string
"@python3_10_12_host//:python".

protobuf 29.5 (bazel/repos.bzl)

Upgraded to protobuf 29.5 (switching from a git_repository to a more
reproducible http_archive). This required downstream changes:

• rules.bzl: ProtoInfo moved from @rules_proto//proto:defs.bzl
  to @com_google_protobuf//bazel/common:proto_info.bzl. Also fixed a
  latent output-directory bug exposed by Bazel 7: output_files[0].path
  for directory-typed outputs already includes the workspace root for
  external repos, so the old code that appended workspace_root again
  was doubling the path and breaking protoc invocations when this rule
  is consumed from an external workspace.
• tests/python/BUILD.bazel: py_proto_library moved to
  @com_google_protobuf//bazel:py_proto_library.bzl; attribute renamed
  from srcs to deps.
• tests/cpp/BUILD.bazel: cc_proto_library moved to
  @com_google_protobuf//bazel:cc_proto_library.bzl.
• requirements.in / requirements_lock.txt: The Python protobuf
  runtime must match the gencode version. Protobuf 29.5 produces gencode
  for runtime >=5.29.5; the old >=3 lower bound caused a
  VersionError at import time.

Test plan

• bazel test //... in this repo passes
• bazel test //... in the monorepo passes (consumes this as a submodule)
• PR for dependent repos (stout, public, mono) is also merged

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