fix(contract): require configured bridge addresses

[h1065153539-create]

Closes #510 by distinguishing chains that merely exist in storage from chains that have an explicitly configured bridge.

Changes:

• add bridge_configured to ChainConfig
• keep default chain configs unconfigured until the bridge is set
• hide bridge addresses and active chain listings until configured
• mark bridge configuration as explicit when update_bridge_address is called
• add tests covering the default and configured states

Local Rust verification was attempted, but the Windows toolchain on this machine is missing link.exe, so full build/test execution is blocked by environment.

[Xhristin3]

@h1065153539-create — issue #510 isn't currently assigned to anyone, so I can't merge this one per repo policy. Could you ask a maintainer to assign it to you (or to whoever picks it up), then I'll merge. 👍