fix(urma): address review feedback on Bazel docs and mock builds

[cw20050111-prog]

What problem does this PR solve?

Follow-up for apache#3428. This PR applies the effective fixes made after the reviewed urma_transport branch state, targeting the reviewer feedback on URMA Bazel documentation/build behavior and mock usage.

Reviewer feedback addressed:

• Add Bazel-specific URMA build instructions.
• Clarify that DOWNLOAD_URMA_HEADERS is a CMake-only option and document the Bazel behavior for UMDK headers.
• Add an independent opt-in switch for the URMA link-time mock so normal URMA builds do not silently fall back to mock mode.

Issue Number: Related to apache#3401 and apache#3428

What is changed and the side effects?

Changed:

• Document Bazel URMA build support and its current limitations in both English and Chinese docs.
• Keep Bazel/Bzlmod UMDK resolution reproducible by using the pinned UMDK commit already introduced for feat: add URMA transport support apache/brpc#3428, and strip UMDK's upstream src/urma/BUILD.bazel so the exported header glob works as intended.
• Add WITH_URMA_MOCK / --with-urma-mock as an explicit opt-in for the link-time mock when liburma is unavailable.
• Make example CMake dependency discovery link liburma by best effort from the shared example helper, avoiding duplicate lookup in urma_performance.
• Add Linux CI coverage for WITH_URMA=ON with the explicit mock path and fix the mock for the C++14 build baseline.
• Let the Linux/Mac build workflows run for urma_transport PRs so this review branch shows the full CI signal.

Side effects:

• Existing builds keep URMA disabled by default.
• WITH_URMA=ON without liburma now fails fast unless WITH_URMA_MOCK=ON is explicitly set. This avoids producing a binary that appears URMA-capable but only links the mock.
• The mock path remains available for CI and no-hardware test environments.

Verification

• git diff --check origin/urma_transport..HEAD passed.
• Parsed .github/workflows/ci-linux.yml and .github/workflows/ci-macos.yml successfully with Ruby YAML.
• CMake configured successfully with WITH_URMA=ON, WITH_URMA_MOCK=ON, cached UMDK headers, and local OpenSSL.
• A local macOS build reached URMA compilation and stopped at sys/eventfd.h, which is Linux-only; the added Linux workflow covers this path on Ubuntu.
• GitHub Actions are now attached to this PR: Linux build/test, macOS build, and License Check.

Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.