Improve RadiopharmaceuticalInformationSequence exception handling to surface data quality issues

[Copilot]

Addresses review feedback on #92 regarding silent exception swallowing in DICOM metadata loading. The previous implementation caught both AttributeError and IndexError, masking potential data quality issues when the sequence exists but is empty.

Changes

• Replace try-except with explicit hasattr() and len() checks following pattern established in cycle_loader.py, dicom_ingest.py
• Emit warning when RadiopharmaceuticalInformationSequence exists but is empty (likely data quality issue)
• Provide specific messages for each failure case (missing attribute, empty sequence, missing dose field)
• Maintain backward compatibility: all paths default to 7400 MBq as expected by existing tests

Before

try:
    injected_activity = (
        dicom_slices[0]
        .RadiopharmaceuticalInformationSequence[0]
        .RadionuclideTotalDose
    )
except (AttributeError, IndexError):
    print("Injected activity not found in DICOM header. Using default: 7400 MBq")
    injected_activity = 7400.0

After

injected_activity = None

if hasattr(dicom_slices[0], "RadiopharmaceuticalInformationSequence"):
    rp_seq = dicom_slices[0].RadiopharmaceuticalInformationSequence
    if len(rp_seq) > 0:
        try:
            injected_activity = rp_seq[0].RadionuclideTotalDose
        except AttributeError:
            print("RadiopharmaceuticalInformationSequence found but RadionuclideTotalDose is missing.")
    else:
        print("Warning: RadiopharmaceuticalInformationSequence is empty. This may indicate a data quality issue.")

if injected_activity is None:
    print("Using default injected activity: 7400 MBq")
    injected_activity = 7400.0

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[carluri]

Makes sense