feat: conditionally inject Arduino.h for ino preprocessing

[zackees]

Summary

• Make .ino preprocessing inject #include <Arduino.h> only when the active core/variant include roots contain Arduino.h.
• Keep direct sketch scans header-agnostic, and pass include roots through the full scan and STM32 Arduino mbed path.
• Add tests for header-present/header-absent output and no-rewrite stability in both modes.

Closes #646

Tests

• soldr cargo fmt
• soldr cargo test -p fbuild-build source_scanner
• soldr cargo test -p fbuild-build script_runtime::tests::test_shim_medium_default_environment_linkflags -- --exact
• soldr cargo clippy -p fbuild-build --all-targets -- -D warnings
• soldr cargo test -p fbuild-build

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 2452015b-da5c-4d5f-ae29-1ecfa10c25ce

📥 Commits

Reviewing files that changed from the base of the PR and between d25c060 and 2dc2a37.

📒 Files selected for processing (2)

• crates/fbuild-build/src/source_scanner.rs
• crates/fbuild-build/src/stm32/orchestrator/arduino_mbed.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/646-conditional-arduino-include

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