fix[codegen]: assert staticcall in constant context

[banteg]

What I did

Fixes #5060.

Legacy external-call lowering carries a codegen-layer sanity assert that a constant (view/pure, or range-expression) context can only emit STATICCALL (vyper/codegen/external_call.py: assert use_staticcall, "typechecker missed this"). The Venom external-call lowering derived use_staticcall from the target's mutability but never cross-checked it against the enclosing context's constancy — a frontend miss would silently emit CALL instead of failing loudly in codegen.

How I did it

Added the equivalent two-line assert in Expr._lower_external_call, immediately after use_staticcall is derived and before the CALL/STATICCALL dispatch. VenomCodegenContext.is_constant() mirrors legacy Context.is_constant() semantics (per-function constancy from view/pure mutability, plus forced constancy inside range expressions). The assert message matches legacy verbatim for greppability.

How to verify it

The assert is unreachable by construction (semantic analysis rejects state-mutating external calls in constant contexts), matching the untested legacy assert — this is defense-in-depth parity. Suites pass: tests/unit/compiler/, tests/functional/codegen/calling_convention/ and test_interfaces.py under --experimental-codegen.

Commit message

legacy external-call lowering asserts at codegen time that a constant
(view/pure or range-expression) context only emits staticcall. the
venom lowering derived `use_staticcall` from the target's mutability
but never cross-checked it against the enclosing context's constancy,
so a frontend miss would silently emit CALL instead of failing loudly.

add the equivalent assert to the venom external-call lowering,
matching the legacy message.

the assert is unreachable by construction; this is defense-in-depth
parity.

Description for the changelog

Fix: Venom external-call lowering asserts that constant contexts only emit STATICCALL, matching legacy defense-in-depth.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.93%. Comparing base (242a4fa) to head (359fdba).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5090   +/-   ##
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  Coverage   92.93%   92.93%           
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  Files         188      188           
  Lines       27840    27843    +3     
  Branches     4831     4832    +1     
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+ Hits        25874    25877    +3     
  Misses       1315     1315           
  Partials      651      651           

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