Refactor explain to a common help

[hadia206]

Extracts shared logic from explain.py into a new pydough/exploration/_common.py module, laying groundwork for the upcoming explain_llm API.

No behavior changes. The user-facing output of pydough.explain() is identical.

Changes:

• Replaces inline term-gathering loop with extract_terms()
• Replaces inline AND-condition splitting with extract_conditions()
• Replaces bare try/except around qualify_node with qualify_safely(), which returns (result, error) instead of raising
• Adds find_source_collection() for walking the preceding_context chain to the root TableCollection