fix!: reject invalid filter input and make the parser correct & concurrency-safe

[christiangda]

Summary

The filter parser was accepting several classes of invalid input and producing a misleading AST, which undermines its role as a query validator. This PR fixes the core correctness bugs, removes a data race, and modernizes a few idioms. All findings were reproduced with tests before fixing.

Bugs fixed

1. Trailing / incomplete input silently accepted. Parse stopped at the first complete expression and ignored the rest. Parse now requires the entire input to be consumed.

   Input	Before	After
   first_name = 'John' garbage	✅ accepted	❌ error
   age > 30 40	✅ accepted	❌ error
   first_name = 'John')	✅ accepted	❌ error
   1 = 1	✅ accepted	❌ error

2. FilterParser was not safe for concurrent use — confirmed data race under go test -race. Per-parse state (lexer, currentToken, errors) moved off the shared struct into a per-call filterParseState. SortParser/FieldsParser were already stateless; now all three can be created once and shared.

3. Dead IsNot fields → misleading AST. NOT IN / NOT BETWEEN / NOT SIMILAR TO / NOT DISTINCT FROM were wrapped in a UnaryOperatorNode{NOT} around a node whose own IsNot was always false. They now set IsNot = true on the operator node itself (consistent with how RegexMatchNode already worked); NOT LIKE uses TokenOperatorNotLike. Type() and String() now honor negation.

4. DISTINCT FROM <value> discarded the value. Added DistinctNode.Value, which now holds the compared value.

5. Stray + lexed to an empty-type token that slipped past the illegal-token check. It now lexes to ILLEGAL.

Idiom / modernization

• map[string]struct{} sets instead of map[string]any (the previous "avoid allocating" comment was backwards) across all three parsers.
• FilterParser.Parse returns errors.Join(...) of *QFVFilterError so callers can use errors.As/errors.Is.
• go directive bumped to 1.25.0 (already pending in the working tree); range-over-int in new tests.

Tests

• New filter_parser_regression_test.go: trailing tokens, DISTINCT value retention, concurrent reuse (run under -race), and + lexing.
• Existing AST tests updated to assert the new negation representation.
• go test -race ./... green; go vet clean; gofmt clean; coverage 80.1%.

⚠️ Breaking changes

This is a pre-1.0 breaking change (suggested tag: v0.1.0). Full migration guide is in the README under "Breaking Changes & Migration". In short:

• Negated-operator AST shape changed (IsNot on the node; NOT LIKE → TokenOperatorNotLike).
• DistinctNode gained a Value field.
• Parse returns a joined error instead of one formatted string.
• Inputs previously accepted by mistake are now rejected.

Code that only calls Parse and checks err != nil needs no changes (beyond fixing inputs that were never actually valid).

🤖 Generated with Claude Code