cargo_build_script_runner: parse cargo::metadata key/value pairs

[sebas-zip]

Summary

Fix cargo::metadata=KEY=VALUE handling in cargo_build_script_runner so metadata keys are propagated in Cargo-compatible form.

Before:

• cargo::metadata=version_1_10_0=1
• produced DEP_<links>_METADATA=version_1_10_0=1

After:

• same input produces DEP_<links>_VERSION_1_10_0=1

Changes

• Add a "metadata" parser branch in BuildScriptOutput::new.
• Split metadata payload on first = and emit DepEnv("<KEY>=<VALUE>").
• Preserve fallback behavior for malformed metadata payloads.
• Add unit test: metadata_directive_maps_to_dep_env_key_value.

Motivation

Downstream links users (for example hdf5-metno) expect specific DEP_* metadata keys (DEP_HDF5_VERSION_*, DEP_HDF5_HAVE_*).

Testing

• bazel --batch test //cargo/private/cargo_build_script_runner:test --remote_executor= --remote_cache= --bes_backend= --noshow_progress --color=no --curses=no

bazel test //test/cargo_build_script/metadata_dep_env:all are end to end integration tests

Closes #3876.

[UebelAndre]

Thanks! Could you also implement an end-to-end test somewhere in //test/cargo_build_script that defines a cargo_build_script that emits this type of dep env and a consuming rust_test or other Rust rule that confirms the data is correct?

[UebelAndre]

Looks great, thank you so much!

[sebas-zip]

thanks so much. I just tried to do some legacy testing of the old method too. I rebased to clean up the commits since I wasn't sure how commits are squashed to main. do you want to do another review? or okay to merge?

[UebelAndre]

Looks good to me! Thank you for the additional test case!