fix(cli): admin payout-issue reports success even when payout_bounty loses the amount

[ebios-star]

Summary

Closes #844.

`IssueCompetitionContractClient.payout_bounty` returns three different values:

Return	Meaning
positive int	success with known amount
`0`	success but the pre-payout `get_issue` couldn't reach the chain (RPC flake / dropped websocket), so the function lost track of the bounty even though the extrinsic succeeded
`None`	extrinsic failed

`gitt admin payout-issue` collapsed the last two cases with `if result:` and printed "Payout failed." — exiting non-zero — when the on-chain state had actually paid out. Operators retried the call and hit the contract's already-Completed revert, reinforcing the wrong belief.

The CLI already reads the issue once before submitting (line 139), so when `payout_bounty` returns `0` we can fall back to that locally-known bounty for the success message instead of mis-labelling the round as a failure.

After this change:

• `if result is not None` gates success/failure — `0` is correctly recognized as success.
• The success message prefers the contract client's amount when it's positive, and falls back to `issue.bounty_amount` when it's `0`.
• The contract_client API stays unchanged.

Related Issues

Closes #844.

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other

Testing

• New `TestAdminPayoutSuccessWithUnknownAmount` covers all three cases:
  • `payout_bounty` returns 0 → success message uses local bounty (`42.0000 ALPHA`)
  • `payout_bounty` returns None → "Payout failed." + exit 1 (existing contract preserved)
  • `payout_bounty` returns positive → uses that amount (forward-compatible)
• `uv run --extra dev pytest tests/` — 646 tests pass (643 baseline + 3 new).

Checklist

• Code follows project style guidelines
• Self-review completed
• Changes are documented (if applicable)

[ebios-star]

@anderdc @LandynDev — friendly ping for review when you have a moment.

Closes #844. gitt admin payout-issue was reporting "Payout failed." + exit 1 when the on-chain transaction had actually succeeded — because payout_bounty returns 0 (success but unknown amount) when its internal get_issue flakes mid-call, and the CLI's if result: truthiness check collapsed that with None (genuine failure). Operators retried the call and hit the contract's already-Completed revert.

Switched to if result is not None, falling back to the bounty we already read pre-payout when the contract client lost track. contract_client API stays unchanged. 3 new regression tests covering all three return values.

Happy to rebase if needed.