Add buf linting workflow with Bazel integration via Azure Pipelines

[mmorel-35]

Implements buf-based protobuf linting with Azure Pipelines CI and Bazel integration using rules_buf v0.5.2. Supports both bzlmod (MODULE.bazel) and WORKSPACE build modes. All proto files are formatted according to buf standards.

Changes

Buf configuration

• buf.yaml: v2 config with STANDARD linting rules, excludes bazel build artifacts
• buf.lock: pins dependencies (googleapis, protoc-gen-validate, cel-spec)
• Applied buf formatting to all proto files (import ordering, spacing, multi-line field options)

Bazel integration

• MODULE.bazel: added rules_buf v0.5.2 dependency for bzlmod
• bazel/repositories.bzl: added rules_buf to WORKSPACE dependencies
• bazel/repository_locations.bzl: pinned rules_buf v0.5.2 release tarball with SHA256
• bazel/dependency_imports.bzl: initialized rules_buf toolchains with buf v1.47.2
• bazel/api_build_system.bzl: integrated buf_lint_test into xds_proto_package() macro
• BUILD: added repository-wide buf_lint_test and buf_format targets

Each proto package now automatically gets a pkg_buf_lint_test target:

# In any BUILD file using xds_proto_package()
xds_proto_package()  # Generates pkg, pkg_cc_proto, pkg_go_proto, pkg_buf_lint_test

Buf rules available

• buf_lint_test: Linting for proto files (runs automatically in CI via bazel test //...)
• buf_format: Format proto files with bazel run //:buf_format (available but not enforced in CI)

CI workflow (Azure Pipelines)

• Uses existing bazel test --config=ci //... command in ci/check.sh
• All buf_lint_test targets run automatically through Bazel
• Buf binary provided by rules_buf toolchain (no manual installation needed)
• Follows best practices from buf.build documentation

Closes #47
Closes #45