fix: install composer before user build steps

[AaronFeledy]

Fixes #211 — composer silently missing when build_as_root fails.

Moves composer installation from build_internal to build_as_root_internal so it runs before user build steps. Adds build-fail example test to validate.

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Note

Medium Risk
Changes PHP service build-step ordering by moving Composer installation earlier in the root build phase, which can affect existing projects that relied on previous sequencing. Adds a CI example to catch regressions around failed build_as_root steps.

Overview
Ensures the requested Composer version is installed via build_as_root_internal (instead of being the first build_internal step), so Composer is available even if a user build_as_root command fails.

Adds a new examples/build-fail Leia test case and includes it in the PR workflow matrix to validate Composer remains installed when build_as_root exits non-zero.

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