feat(coc): add tunnel ID field to Add Agent dialog

[plusplusoneplusplus]

Summary

• Adds a conditional Tunnel ID input to the Add Agent dialog, matching the edit form behavior
• The field is shown only when the address URL ends with .devtunnels.ms
• Updates the onAdd callback signature from (address, name?) to (address, name?, tunnelId?)
• Resets tunnel ID state on successful submit
• Adds 15 tests covering conditional rendering, onAdd wiring, error handling, and submit button state

Previously, users had to add the agent first (without a tunnel ID) and then click the pencil edit button to set one. Now the field is available directly in the Add dialog.

Test plan

• Add an agent with a .devtunnels.ms address — tunnel ID field appears
• Add an agent with a local address — tunnel ID field absent
• Fill in tunnel ID and verify it is saved on the new agent
• Change address from devtunnels to localhost — tunnel ID field disappears
• All 15 new unit tests pass

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