fix: metrics SQL compiles IS FALSE / IS NOT FALSE identically to IS TRUE

[nishantmonu51]

parseIsTruthOperation in the metrics SQL filter parser never inspected ast.IsTruthExpr.True, which distinguishes IS TRUE from IS FALSE. As a result, flag IS FALSE compiled to the exact same expression as flag IS TRUE (and IS NOT FALSE the same as IS NOT TRUE), silently inverting boolean filters in metrics SQL queries, canvas dimension_filters, and SQL-based where expressions.

• Branch on node.True so IS FALSE compiles to eq(expr, false) instead of eq(expr, true).
• Compare against a typed boolean value instead of the string 'TRUE', which relied on implicit string-to-boolean casts that are not reliable across ClickHouse/Druid.
• IS NOT TRUE / IS NOT FALSE now compile to expr != <bool> OR expr IS NULL, matching SQL semantics where the negated forms include NULL rows (a plain != excludes them).
• Added parser tests for all four combinations; all four fail against the previous implementation.

Steps to reproduce manually:

1. In a Rill project with a metrics view mv that has a boolean dimension flag, run a metrics SQL query (e.g. via a metrics_sql resolver in a custom API, or a canvas widget's dimension_filters):

   SELECT flag, count FROM mv WHERE flag IS FALSE

2. Before this fix, the result contains only rows where flag is true; the same query with IS TRUE returns identical results. After the fix, IS FALSE returns the false rows.
3. The inversion is also visible at the parser level:

   expr, _ := metricssql.ParseFilter("flag IS FALSE")
   b, _ := json.Marshal(expr)
   fmt.Println(string(b))
   // before: {"cond":{"op":"eq","exprs":[{"name":"flag"},{"val":"TRUE"}]}}  (identical to IS TRUE)
   // after:  {"cond":{"op":"eq","exprs":[{"name":"flag"},{"val":false}]}}

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[nishantmonu51]

@codex: review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Can't wait for the next one!

Reviewed commit: c48fd4bae2

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[begelundmuller]

LGTM