Skip invalid Slither constants during value seeding

[0xjoma]

Description

Fixes a startup panic in Slither-backed value seeding.

SeedFromSlither ignored the ok result from big.Int.SetString in both the integer and address branches. If Slither emits a numeric- or address-typed constant whose value is not a valid base-10 string, Medusa passed a nil *big.Int into downstream value-seeding code and panicked: (*big.Int).Neg(nil) for the integer branch and common.BigToAddress(nil) -> b.Bytes() for the address branch.

This change keeps the existing decimal-only parsing behavior and skips constants that cannot be parsed.

Related Issues: Closes #830

Type of Change:

• Bug Fix
• Tests

Testing

Run inside a fresh clone on dev/slither-seed-parse-failure:

go fmt ./...
go vet ./fuzzing/valuegeneration/
go test ./fuzzing/valuegeneration/
go test ./...
golangci-lint run --timeout 5m0s

The new test TestSeedFromSlitherInvalidUintDoesNotPanic panics on stock master at value_set_from_slither.go:19 and TestSeedFromSlitherInvalidAddressDoesNotPanic panics at line 35; both pass after the patch.

Additional Context

0x parsing is intentionally not added in this PR to keep the bug fix separable from a behavior expansion. The existing AST-based seed path in value_set_from_ast.go already handles 0x-prefixed numeric values; whether the Slither-based path should match it is a separate discussion.

Outstanding TODOs

None.

[claude]

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