fix: add nil checks to BinarySource methods

[qj0r9j0vc2]

Summary

• Add nil receiver checks to BinarySource methods to prevent SIGSEGV panic
• IsLocal(), IsGitHubRelease(), IsValid() return false when nil
• MarkValid(), MarkInvalid() are no-ops when nil
• Validate() returns descriptive error when nil

Problem

The upgrade command panics with nil pointer dereference at binary_source.go:149 when BinarySource is nil:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x133c300]

goroutine 1 [running]:
github.com/altuslabsxyz/devnet-builder/internal/domain.(*BinarySource).IsLocal(0x1da21a0?)
        /home/ubuntu/devnet-builder/internal/domain/binary_source.go:149
github.com/altuslabsxyz/devnet-builder/cmd/devnet-builder/commands/manage.runUpgrade(...)
        /home/ubuntu/devnet-builder/cmd/devnet-builder/commands/manage/upgrade.go:294

Root Cause

At upgrade.go:294, selection.BinarySource.IsLocal() is called without nil check. The interactive selection can return nil BinarySource in certain flows.

Test plan

• Added unit test TestBinarySourceNilSafety verifying all methods handle nil safely
• go build ./... passes
• go test ./internal/domain/... passes