Fix out-of-range BC slice for ambiguous tracks past the last BC

[sawenzel]

This fixes a problem in the AOD producer's BC slice assignment for ambiguous tracks.

• BunchCrossings::lower_bound returns {size(), 0} when no BC is at or after the requested one, and fillBCSlice used that pair unchecked.
• A track whose time window starts after the last BC of the timeframe therefore got fIndexSliceBCs[0] equal to the number of BCs, one past the end of the BC table.
• The same pair sets the time reference, where 0 - mStartIR.toLong() underflows in unsigned arithmetic, so fTrackTime was written as about -4.6e20 ns.
• The binary-search implementation that the accelerated lookup replaced clamped this case to the last BC; fillBCSlice now does the same.
• Seen in LHC26a5a_gp_2025_v10 (Pb-Pb apass1 anchored MC), where 51 of 736770 ambiguous tracks were affected and later crashed the AnalysisQC event selection QA task.

[alibuild]

Error while checking build/O2/fullCI_slc9 for 1d089f7 at 2026-09-23 00:03:

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