fix: Improve error messages for missing main pipeline

[max-sixty]

Summary

Replaces the generic "Missing main pipeline" error with context-aware, beginner-friendly messages:

• Empty queries/comments: No PRQL query entered (self-explanatory, no confusing hints)
• Queries with declarations but no pipeline: PRQL queries must begin with 'from' with helpful hint

Changes

• Modified error generation logic in prqlc/prqlc/src/semantic/lowering.rs to detect query context
• Added comprehensive tests in prqlc/prqlc/tests/integration/error_messages.rs
• All 573 tests pass ✅

Addresses Issue #4909

✅ Not helpful for newcomers → Uses beginner-friendly language
✅ Confusing in Playground → "No PRQL query entered" is self-explanatory
✅ Uses compiler-centric terminology → Removed "main pipeline" from user-facing errors
✅ Doesn't tell what to do → New hint explains queries must start with from

Test Plan

# Empty query
echo '' | prqlc compile
# Output: [E0001] Error: No PRQL query entered

# Query with declarations but no 'from'
echo 'let x = 5' | prqlc compile  
# Output: [E0001] Error: PRQL queries must begin with 'from'
#         ↳ Hint: A query must start with a 'from' statement to define the main pipeline

Closes #4909

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