Card requests filtered by currency

# Instructions — Card requests with currency filter

## ⚠️ Behavior of the currency filter

When `currency` is provided in `get_requests`:
- **Only** requests submitted in that currency are returned
- Requests in other currencies are **entirely excluded** — no FX conversion
- The empty response (`data: []`) does not distinguish "no requests" from "requests exist in other currencies"

Example confirmed live:
- All requests on the account are in EUR
- `currency=GBP` → **0 results** (no warning)
- `currency=USD` → **0 results** (no warning)
- `currency=EUR` → 3 results
- Without currency filter → 3 results (all currencies, FX to functional currency)

## When to use the currency filter

**Valid case**: the user explicitly wants requests submitted in a specific currency.
```json
{ "companyId": "...", "currency": "GBP", "minAmount": 100 }
```
→ Always display: *"Results limited to requests submitted in GBP. Requests in EUR, USD, or other currencies are not included."*

**Case to avoid**: the user wants "all requests > 100 EUR"
→ Do not pass `currency=EUR` with `minAmount=100` if requests in GBP exist — they would be excluded
→ Without `currency`: `minAmount/maxAmount` filter in **functional currency** via stored FX (all currencies included)

## Recommended pattern: always inform

```
Results: N requests found in [currency].
⚠️ Requests submitted in other currencies are not included in this result.
For a view across all currencies, rerun without the currency filter.
```

## Step 1 — Call without currency filter (full view)

```json
{
  "companyId": "...",
  "state": "pending"
}
```
→ Returns all requests, `minAmount/maxAmount` in functional currency.
→ `amountInFunctionalCurrency` = value comparable across currencies.

## Step 2 — Call with currency filter (specific currency view)

```json
{
  "companyId": "...",
  "currency": "GBP",
  "minAmount": 500
}
```
→ Returns requests submitted in GBP with `requestedAmount.amount >= 500 GBP`.
→ **Always warn** the user that other currencies are excluded.

## Step 3 — Interpreting the results

For each request:
- `requestedAmount`: amount in the submission currency
- `amountInFunctionalCurrency`: equivalent in functional currency (EUR for this account)
- `isForeignCurrency=true` → submitted in a currency different from the functional one

## Example prompt

- "Show pending card requests > 1000 EUR" → without currency, minAmount=1000
- "Which requests were submitted in USD?" → currency=USD + inform about the exclusion
- "All requests > 500, across all currencies" → without currency, minAmount=500 in functional currency