fix(core): return valid slot count from scan

[ujjwalredd]

Problem

eos_slot_scan_all() is documented to return the number of valid firmware slots but always returned EOS_OK (zero). The existing slot-manager test reimplemented every public slot API inside the test executable, so it tested the fake implementation rather than core/slot_manager.c. Release builds also disabled its standard assert() checks through NDEBUG.

Approach

• Count successful A/B verification results and return a value from 0 to 2.
• Replace public-API fakes with HAL and image-verification boundary stubs.
• Use assertions that remain active in Release builds.
• Cover zero, one, and two valid slots; state/header/version queries; integrity/signature failures; invalid recovery-slot requests; and erase success/failure.

No public signature or dependency changes are introduced.

Validation

• Regression before fix: failed as expected at the one-valid-slot count.
• Release test_slot_manager: 6/6 passed.
• ASan/UBSan test_slot_manager: 6/6 passed.
• All independently buildable C tests except the baseline-blocked recovery target: 13/13 passed.
• Python tests excluding the unrelated runner failure: 9 passed.
• Config generation from a representative boot layout: passed and produced both generated files.
• git diff --check: passed.
• Independent review: approved with no findings.

Additional considerations and limitations

The unmodified baseline full build already fails while linking test_recovery because eos_boot_log_append, eos_boot_log_get_head, and eos_boot_log_read are unresolved. The full Python suite also has one unrelated existing failure: run_all_tests.run_tests() raises SystemExit while its test expects a return value. This PR does not hide or expand into either issue. Validation was host-based; no hardware flashing was performed.

GitHub Actions created build, simulation, CodeQL, and assignment runs for this PR, but all are currently action_required pending maintainer approval.

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