Preserve erased flags on synthetic context-function parameters

[tanishiking]

fix #25725

problem
For normal parameters, erasure removes erased parameters from both the method type and the erased tree:

def direct(using erased p: Proof, x: Int): String =
  "value: " + x

// [[syntax trees at end of erasure]]
def direct(using x: Int): String =
  "value: " + Int.box(x)

However, parameters coming from a context-function result were not treated the same way. For example:

def mixed: (erased Proof) ?=> Int ?=> String =
  s"value: ${summon[Int]}"

// after typer: desugared
def mixed:
    PolyFunction {
      def apply(using erased x$1: Proof): (Int) ?=> String
    }
   = (using x$1: Proof) => (using contextual$1: Int) => ...

// erased to a tree that still kept the erased parameter:
def mixed(using x$1: Proof, contextual$1: Int): String =
  "value: " + Int.box(contextual$1)

This produced a mismatch between the symbol info and the erased tree.

On the type side, TypeErasure correctly drops erased parameters from the MethodType, so mixed.symbol.info becomes (using Int)String, and the backend emits the JVM descriptor (I)Ljava/lang/String;.

On the tree side, the erased DefDef still contains x$1, so the backend assigns local slots as if both parameters were present: 0 = this / 1 = x$1 / 2 = contextual$1
As a result, the body loads contextual$1 with iload_2, even though the descriptor contains only one argument. That mismatch results in verifier error.

Root cause

The root cause is that erased flagfor synthetic context-function parameters was stored only in FunctionWithMods.erasedParams, but was not stored onto the synthetic parameter ValDefs themselves. makeContextualFunction created parameters with Given | Param, but not Erased. Later, Erasure.skipContextClosures decides whether to keep flattened context-result parameters by checking param.symbol.is(Erased), so these parameters were incorrectly seen as non-erased and left in the tree.

fix

This commit fix by keep function-parameter metadata (erasedness) on synthetic parameters consistently. We introduce a shared helper in Desugar and use it from both makeContextualFunction so that parameter-level flags such as given/implicit and erased are applied to the generated ValDefs before they are turned into symbols.

With that change, synthetic context-function parameters carry Erased to the Erasure phase, the erased tree matches the erased method type, and the backend assigns locals consistently.

How much have you relied on LLM-based tools in this contribution?

moderately use Codex gpt-5.4

How was the solution tested?

testCompilation i25725

Additional notes

[tanishiking]

Requesting review to @natsukagami based on the commit history #16507 🙏

[natsukagami]

LGTM

[tanishiking]

Thanks!