fix: require USD accounts for Ads launch and monitoring

[hamitarsln]

What and why

Launch and monitor use USD budgets and costs, while Google Ads uses the account's currency. Require a confirmed USD account before campaign drafting or monitoring. Other or missing currencies stop the flow.

The first account read also saves its failure reason for the final message. A billing refusal stops without retrying; an unconfirmed provider result keeps its existing retry behavior.

Google's budget field reference defines the amount in the account's currency.

Verification

The missing currency guard was reproduced twice. Three follow-up checks also failed because account-read causes disappeared from the final message.

After the fix, 141 related tests passed with a disposable database, no skips. Both PRs #394 and #400 passed 185 tests together, including one-attempt replay, no model calls or campaign changes on a refused account read. Their combined code merges without conflict.

Ruff, formatting and diff checks passed.

Limits

Synthetic providers only; no live Ads account was used. USD is the supported currency. No exchange-rate conversion is added. Live account access and production deployment still need a separate check.

[Copilot]

🟢 Approval recommended

The guards consistently stop unsafe flows and have focused coverage for launch, monitoring, and read failures.

0 open findings

What changed in this PR

Adds USD currency validation to prevent Google Ads budgets and costs from being misinterpreted.

Changes:

• Blocks launch and monitoring unless the account confirms USD.
• Preserves account-read failure reasons.
• Adds currency guard tests and documentation.

File	Description
src/​tin_lite/​paid_ads_launch.py	Adds the unsupported-currency launch outcome.
src/​tin_lite/​paid_ads_monitor_activities.py	Validates currency before monitoring.
tests/​test_paid_ads_currency.py	Tests unsupported and unavailable currencies.
tests/​test_paid_ads_monitor_activities.py	Adds account fixtures and updates read counts.
docs/​paid-ads-launch-implementation.md	Documents the USD requirement.

🧠 Review effort: Balanced

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[demegire]

Closing in favour of #402, which supports other account currencies instead of requiring USD: one ECB rate is pinned at launch and amounts are converted onto Google's minimum unit, and currencies Tin cannot price stop with a named reason. It does not include this PR's account-read failure reasons in the monitor's final message; that can follow separately if still wanted.