feat(eap): Add NULL-safe division and ConditionalFormula support for formulas

[phacops]

Summary

Adds two features to support complex formula calculations in EAP queries:

1. NULL-safe division

When a BinaryFormula with OP_DIVIDE has a default_value specified, the division now safely handles NULL and zero divisors:

-- Before: returns NULL when divisor is NULL or zero
count(*) / ((now - min(timestamp)) / 3600)

-- After: returns default value (e.g., 0.0) when divisor is NULL or zero  
if(isNull(divisor) OR divisor = 0, default_value, numerator / divisor)

This enables calculations like count / hours_since_first_seen where hours_since_first_seen derives from min(timestamp).

2. ConditionalFormula support (waiting for sentry-protos PR #173)

Adds support for Column.ConditionalFormula to express conditional expressions:

if(min(timestamp) < one_week_ago, count/WEEK_IN_HOURS, count/hours_since_first_seen)

The code is implemented and ready but dormant until the proto is released. Uses _has_proto_field() helper to safely check for proto fields that don't exist yet.

Changes

• resolver_trace_item_table.py:

  • Added _has_proto_field() helper for safe proto field checking
  • Added COMPARISON_OP_TO_EXPR mapping for conditional formula operators
  • Modified _formula_to_expression() for NULL-safe division
  • Added _conditional_formula_to_expression() using existing if_cond() DSL
  • Updated _column_to_expression() to handle conditional_formula
  • Updated _get_reliability_context_columns() for conditional formulas

• proto_visitor.py:

  • Updated ColumnWrapper.accept() to traverse conditional formula children

Test plan

• test_hourly_rate_with_dynamic_divisor_using_aggregate passes (NULL-safe division)
• test_conditional_rate_based_on_aggregate_first_seen skips until proto available
• All existing tests pass (91 passed, 1 skipped, 2 xfailed)
• Linting and mypy pass

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