builtin: guard array push len panics

[medvednikov]

Fixes #27334.

What changed

• Guard the array.push and array.push_noscan negative-length sanity checks with $if !no_bounds_checking.
• Keep the max_int length guards unconditional so required := a.len + 1 cannot overflow when bounds checking is disabled.
• Add a C-output regression that verifies -prod -no-bounds-checking generated C omits the negative-length panic paths while retaining the max-length overflow guards for both regular and noscan array pushes.

Why

The negative-length checks are bounds-checking sanity paths and can be compiled out consistently with existing array access checks. The max_int checks are capacity-overflow guards, so they remain active even with -no-bounds-checking.

Validation

• ./v -g -keepc -o ./vnew cmd/v
• ./vnew test vlib/builtin/array_test.v
• ./vnew -silent vlib/v/gen/c/coutput_test.v

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep the max_int push guard enabled

When compiling with -no-bounds-checking and appending to an array whose len is already max_int (for example a very large byte array or one constructed through unsafe/C interop), this wrapper removes the only guard before required := a.len + 1; that addition then overflows before the capacity check and the subsequent write uses the old max_int offset, causing UB/memory corruption instead of the previous panic. -no-bounds-checking disables index checks, but other capacity-overflow guards such as push_many/insert_many remain unconditional, so the max length guard should stay outside the $if (or be replaced with an overflow-safe computation); the same applies to push_noscan.

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