fix: bind Int128/UInt128/Int256/UInt256 (big.Int) params as numeric values

[polyglotAI-bot]

Description

Fixes #1917.

*big.Int / big.Int are the Go types behind ClickHouse Int128, UInt128, Int256 and UInt256. In the client-side SQL bind rewrite (?, $1, @name), formatValue() matched *big.Int on the case fmt.Stringer: arm and emitted it as a quoted string literal ('170141183460469231731687303715884105727'). The server then saw a String — toTypeName(?) returned String instead of a number — so type-inference contexts (arithmetic, function dispatch, toTypeName) broke. A big.Int passed by value was worse still: it fell through to fmt.Sprint and produced garbage struct text.

Emitting a bare decimal is not enough: ClickHouse infers an integer literal wider than 64 bits as Float64, losing precision (a WHERE on an Int128 column then matches nothing). The fix instead wraps the exact decimal in the narrowest wide-integer conversion that holds it — toInt128('..'), toUInt128('..'), toInt256('..') or toUInt256('..') — mirroring how the driver already binds times as toDateTime(...) and floats as cast(..., 'Float64'). This keeps both the exact value and an integer type. Server-side {name:Type} query parameters already declare the type, so there the bare decimal is sent unchanged.

Changes

• bind.go: handle big.Int and *big.Int in formatValue() before the fmt.Stringer arm. New helpers formatBigInt / bigIntConvFunc pick the narrowest of Int128/UInt128/Int256/UInt256 that holds the value, or return an actionable error if it fits none. nil *big.Int → NULL.
• bind_test.go: TestFormatBigInt (magnitudes, spill boundaries, out-of-range, both format modes, value + pointer), TestBindBigInt (all three placeholder styles, arrays, tuples, maps, nil, and contrast cases that must stay quoted: string, another fmt.Stringer, decimal.Decimal), TestBindBigIntQueryParameter (server-side {name:Type} path).
• tests/issues/1917_test.go: integration test over Native + HTTP — parameter binds as a wide integer, round-trips exactly, matches an Int128 column, and works via server-side query parameters.
• tests/std/1917_test.go: the same round-trip through the database/sql surface on both protocols.

Test

Verified against ClickHouse 26.5 that the new tests fail on main (*big.Int → '…' → String; big.Int value → fmt.Sprint garbage) and pass with the fix. toInt128('…') etc. confirmed to yield the exact value and the expected wide-integer type on the server. gofmt/go vet clean on changed files; full root-package unit suite green.

Pre-PR validation gate

• Deterministic repro confirmed (tests fail on unpatched bind.go, pass with fix)
• Root cause documented above
• Fix targets the root cause
• Test fails without fix, passes with fix
• No existing tests weakened or broken
• Convention compliance (AGENTS.md / CONTRIBUTING.md — CHANGELOG.md is auto-generated, not hand-edited; regression test under tests/issues/; t.Cleanup + version-gated server-side subtest)