fix: use build.project_name for bootloader artifacts

[officialasishkumar]

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• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features) - not applicable; this restores existing artifact naming behavior for custom build.project_name values.
• UPGRADING.md has been updated with a migration guide (for breaking changes) - not applicable; this is not a breaking change.
• configuration.schema.json updated if new parameters are added. - not applicable; no configuration parameters are added.

What kind of change does this PR introduce?

Bug fix.

What is the current behavior?

Fixes #3108.

When compile is run with a custom --build-property build.project_name=..., the main output artifacts use the custom project name, but the bootloader merge step still looks for artifacts named after the original sketch file. As a result, .with_bootloader.hex and .with_bootloader.bin are not generated for the custom project name.

What is the new behavior?

The bootloader merge step uses the resolved build.project_name, falling back to the sketch main filename only when the property is unavailable. This produces the expected custom-named bootloader artifacts alongside the other custom-named build outputs.

Does this PR introduce a breaking change, and is titled accordingly?

No.

Other information

Validation run locally:

• go test ./internal/arduino/builder -run TestMergeSketchWithBootloaderUsesBuildProjectName -count=1
• go test ./internal/arduino/builder -count=1
• go build
• go test ./internal/integrationtest/compile_1 -run '^TestCompile$/^WithCustomBuildPath$/^OutsideSketchWithBuildProjectName$' -count=1 -timeout 20m

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