Fix CI: add Rust compression mocks and build steps

- Add cargo command mocks to omnibus unit tests so they properly
  simulate the Rust compression build step
- Add Rust compression build to tests_serverless-init and
  tests_serverless-init-logs CI jobs before running go test

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: blt

.gitlab/build/source_test/linux.yml

@@ -134,6 +134,7 @@ tests_serverless-init:
     - when: on_success
   script:
     - !reference [.retrieve_linux_go_deps]
+    - dda inv rust-compression.build --release
     - go test -tags "serverless otlp test" ./cmd/serverless-init/.
 
 tests_serverless-init-logs:
@@ -146,6 +147,7 @@ tests_serverless-init-logs:
     - when: on_success
   script:
     - !reference [.retrieve_linux_go_deps]
+    - dda inv rust-compression.build --release
     - go test -tags "serverless otlp test" ./comp/logs/agent/agentimpl/.
 
 # Check consistency of go.mod file with project imports

tasks/unit_tests/omnibus_tests.py

@@ -79,6 +79,9 @@ def _set_up_default_command_mocks(self):
             (r'grep .*', Result()),
             (r'aws(\.exe)? ssm .*', Result()),
             (r'vault kv get .*', Result()),
+            # Rust compression build commands
+            (r'cargo --version', Result('cargo 1.75.0')),
+            (r'cargo build .*', Result()),
         ]
         for pattern, result in patterns:
             self.mock_ctx.set_result_for('run', re.compile(pattern), result)
@@ -386,6 +389,9 @@ def test_package_parsing(self):
             (r'git .*', Result()),
             (r'aws s3 .*', Result()),
             (r'dpkg --print-architecture', Result('amd64')),
+            # Rust compression build commands
+            (r'cargo --version', Result('cargo 1.75.0')),
+            (r'cargo build .*', Result()),
         ]
         for pattern, result in patterns:
             mock_ctx.set_result_for('run', re.compile(pattern), result)