fix: reject duplicate function parameters with correct error in all contexts

[He-Pin]

Motivation

go-jsonnet issue #873 reports that (function(x, x, x) x)(null, 3, 2)
silently returns 2 instead of rejecting duplicate parameter names.
sjsonnet already rejected duplicates in function expressions, but the
error message was misleading in local bindings and object methods
(e.g. "Expected ")"" or "Expected ":"").

Modification

The params parser correctly detects duplicates via flatMapX +
Fail. However, in bind (local f(x,x)=...) and field
({f(x,x):...}), the optional group .? around params swallowed
the "no duplicate parameter" error and backtracked, producing
misleading errors.

Two changes fix this:

1. In field(): added ~/ (cut) after "(" to commit to the params
   path once "(" is matched: "(" ~/ params(...) ~ ")".
2. In params(): set ctx.cut = true programmatically when a
   duplicate is detected, preventing outer .? operators from
   swallowing the error. Also captures Pos for each parameter to
   position the error at the duplicate parameter name (not at the
   closing paren).

Result

All duplicate parameter contexts now produce the correct error
"Expected no duplicate parameter: x" with accurate position info:

• (function(x, x, x) x) -> ParseError (already worked)
• local f(x, x) = x -> ParseError (was "Expected )")
• { f(x, x): x } -> ParseError (was "Expected :")
• { local f(x, x) = x } -> ParseError (was "Expected )")
• Nested bindings also fixed as a bonus.

Tests pass on Scala 3.3.7, 2.13.18, 2.12.21.

References

• go-jsonnet does not reject duplicate parameter definition google/go-jsonnet#873

Cross-implementation comparison: Duplicate function parameters

Implementation	Detection Phase	All Contexts Covered	Error Message Example
sjsonnet (this fix)	Parse time	✅ All 4 contexts	Expected no duplicate parameter: x
cpp-jsonnet	Static analysis (post-parse)	✅ All 4 (via desugaring)	Duplicate function parameter: x
go-jsonnet (master)	❌ None	❌ Bug — silently accepts	N/A — last argument wins silently
go-jsonnet (PR #876, unmerged)	Parse time	✅ All 4	Duplicate function parameter: a / Duplicate method parameter: x
jrsonnet	Static analysis (post-parse)	✅ All 4 (via desugaring)	local is already defined in the current frame: x

Behavior by test case

Test Case	sjsonnet (fix)	cpp-jsonnet	go-jsonnet	jrsonnet
(function(x, x, x) x)(null, 3, 2)	✅ ParseError	✅ StaticError	❌ Returns 2	✅ StaticError
local f(x, x) = x; f(1, 2)	✅ ParseError	✅ StaticError	❌ Accepted	✅ StaticError
{ f(x, x): x }.f(1, 2)	✅ ParseError	✅ StaticError	❌ Accepted	✅ StaticError
{ local f(x, x) = x, a: f(1, 2) }	✅ ParseError	✅ StaticError	❌ Accepted	✅ StaticError

Key differences

• Detection timing: sjsonnet and go-jsonnet (PR perf: capture parse Position without boxing the offset Int #876) detect at parse time; cpp-jsonnet and jrsonnet detect during a separate static analysis pass after parsing.
• Error message specificity: go-jsonnet PR perf: capture parse Position without boxing the offset Int #876 distinguishes "function parameter" vs "method parameter"; cpp-jsonnet uses "function parameter" uniformly (methods desugar to functions before the check).
• Position accuracy: sjsonnet points to the duplicate parameter name; cpp-jsonnet points to the entire function AST node.
• go-jsonnet master has a known bug (issue #873) where duplicate parameters are silently accepted with "last argument wins" semantics.