fix(shm): fail loudly if CARGO_CFG_TARGET_OS is unset

[YuanYuYuan]

Summary

Follow-up to #2680. That PR replaced commons/zenoh-shm/build.rs's cfg_aliases! macro with a direct CARGO_CFG_TARGET_OS check, using unwrap_or_default() on the env var lookup. Review feedback pointed out that CARGO_CFG_TARGET_OS is always set by Cargo when it runs a build script normally, so silently falling back to an empty string on a missing/unexpected value would misconfigure shm_external_lockfile as false with no diagnostic — the wrong failure mode for a build-time invariant that should never actually be violated under normal use.

Key Changes

• commons/zenoh-shm/build.rs: replace .unwrap_or_default() with .expect(...) on the CARGO_CFG_TARGET_OS lookup, so a nonstandard invocation fails loudly at build time instead of silently misconfiguring BSD/Redox platform detection.

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🏷️ Label-Based Checklist

Based on the labels applied to this PR, please complete these additional requirements:

Labels: bug

🐛 Bug Fix Requirements

Since this PR is labeled as a bug fix, please ensure:

• Root cause documented - see "Summary" above: unwrap_or_default() on CARGO_CFG_TARGET_OS silently computes shm_external_lockfile = false if the var is ever missing, instead of surfacing the problem.
• Reproduction test added - no unit test is meaningful for a build-script env-var-presence assumption; the failure mode this guards against (silent misconfiguration on a genuinely-missing CARGO_CFG_TARGET_OS) has no reliable way to reproduce under normal cargo build, since Cargo always sets it. The fix is a defensive assertion, not a fix for an observed failure.
• Test passes with fix - verified via this PR's own CI run (cargo build/cargo test across the matrix exercise the build script on every job; a passing build confirms expect() doesn't fire under normal Cargo invocation).
• Regression prevention - any future code path that invokes this build script without Cargo's standard environment will now panic immediately with a clear message, rather than silently producing a wrong cfg.
• Fix is minimal - one-line change (.unwrap_or_default() → .expect(...)) in commons/zenoh-shm/build.rs; no other files touched.
• Related bugs checked - grep -rn "unwrap_or_default\|unwrap_or(" commons/zenoh-shm/build.rs confirms this was the only such fallback in the file; no other silent-default pattern exists there.

Why this matters: Bugs without tests often reoccur.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.31%. Comparing base (7583f9a) to head (af4a3eb).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2683   +/-   ##
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  Coverage   74.31%   74.31%           
=======================================
  Files         414      414           
  Lines       62301    62301           
=======================================
+ Hits        46299    46300    +1     
+ Misses      16002    16001    -1     

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[fuzzypixelz]

Thanks!