fix(dashboard): Correctly validate span and tracemetrics aggregates

[sentry]

This PR addresses two validation issues related to dashboard aggregate functions.

Initially, failure_rate was not recognized as a valid aggregate function for span-based widgets, leading to a ValidationError. The initial fix involved adding failure_rate to SPAN_AGGREGATE_FUNCTIONS.

During review, it was noted that this created a duplicate failure_rate entry in DISCOVER_AGGREGATE_FUNCTIONS (which extends SPAN_AGGREGATE_FUNCTIONS and also explicitly lists failure_rate). To resolve this, failure_rate has been removed from SPAN_AGGREGATE_FUNCTIONS, as it is already correctly handled by DISCOVER_AGGREGATE_FUNCTIONS where it is explicitly defined.

Additionally, this PR fixes a ValidationError for tracemetrics aggregates that use the equation| prefix. The isTracemetricsAggregate function was updated to correctly identify and validate these formula-based aggregates, preventing them from being incorrectly flagged as invalid.

This ensures that both failure_rate is correctly validated for spans and equation| prefixed aggregates are accepted for tracemetrics datasets.

Fixes CLI-1CZ

_{Comment @sentry <feedback> on this PR to have Autofix iterate on the changes.}

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Codecov Results 📊

✅ Patch coverage is 100.00%. Project has 5497 uncovered lines.
❌ Project coverage is 81.74%. Comparing base (base) to head (head).

Coverage diff

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- Coverage    81.75%    81.74%    -0.01%
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  Files          428       428         —
  Lines        30106     30106         —
  Branches     19593     19593         —
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+ Hits         24611     24609        -2
- Misses        5495      5497        +2
- Partials      2054      2055        +1

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