fix(pgtype): return error instead of panicking on malformed geometric text

[ultramcu]

Problem

The text scan plans for circle, box, lseg, path and polygon parse the value by calling strings.IndexByte to locate a , or ) separator and then slicing the string — without checking for the -1 "not found" result. Malformed text that is missing an expected separator therefore panics with slice bounds out of range [:-1] instead of returning an error.

This is reachable through the public database/sql Scanner API (e.g. Circle.Scan, or scanning a text-format column into one of these types), so malformed/corrupt input turns into a process-crashing panic rather than a normal scan error.

Repro (panics before this change):

var c pgtype.Circle
_ = c.Scan("<(123456789>") // panic: slice bounds out of range [:-1]

var b pgtype.Box
_ = b.Scan("(1234567890")  // panic

var l pgtype.Lseg
_ = l.Scan("[(123456789")  // panic

var p pgtype.Path
_ = p.Scan("((1234567")    // panic

var g pgtype.Polygon
_ = g.Scan("((1234567")    // panic

point.go already guards this correctly (it uses strings.Cut and returns "invalid format for point"); these five types were missed. This is the same class of malformed-input panic recently fixed for other types (#2507, #2519, #2520, #2522).

Fix

Guard every strings.IndexByte result with an if end == -1 { return fmt.Errorf("invalid format for <type>") } before slicing, matching the existing point.go behavior. Behavior for well-formed input is unchanged; this only turns a panic into a returned error (no public API change).

Tests

Adds (in pgtype/geometric_malformed_test.go, no database required):

• TestGeometricScanMalformedReturnsError — each of the five types returns an error (no panic) on malformed input. Fails before this change (panics), passes after.
• TestGeometricScanValid — well-formed values still parse correctly.

gofmt -l -s, go vet ./pgtype/, and go build ./... are clean.

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Disclosure (per CONTRIBUTING.md): this is an AI-assisted contribution. The bug was surfaced by an automated audit; the fix and tests were written with AI assistance and reviewed and verified locally (fail-before / pass-after).

[jackc]

Merged with some additional fixes found during review.