Title: Add --limit / -n flag to cap returned data points

[Loboston]

• Adds a --limit / -n flag to all CLI commands to cap the number of data points returned
• Fixes _apply_limit to correctly handle CSV string responses (the default format for most endpoints)
• Adds a test suite for _apply_limit covering all response types and CLI flag wiring

Why

Most Alpha Vantage endpoints return large datasets by default. The --limit / -n flag gives users a quick way to preview the most recent N data points without processing the full response.

Testing

Manually tested against TIME_SERIES_DAILY, TIME_SERIES_WEEKLY, TIME_SERIES_MONTHLY, FX_DAILY, FX_WEEKLY,
DIGITAL_CURRENCY_DAILY, and SMA. Also added 5 unit tests covering all code paths, the first test suite in this repo.

Examples
1.

marketdata-cli TIME_SERIES_DAILY AAPL --limit 5

timestamp,open,high,low,close,volume
2026-03-20,247.9750,249.1999,246.0000,247.9900,87139466
2026-03-19,249.4000,251.8300,247.3000,248.9600,34864082
2026-03-18,252.6250,254.9400,249.0000,249.9400,35757874
2026-03-17,252.9550,255.1300,252.1800,254.2300,32361607
2026-03-16,252.1050,253.8850,249.8800,252.8200,32074209

---

marketdata-cli fx_weekly --from_symbol USD --to_symbol EUR -n 5

timestamp,open,high,low,close
2026-03-19,0.87600,0.87620,0.86080,0.86280
2026-03-13,0.86430,0.87630,0.85690,0.87590
2026-03-06,0.84710,0.86720,0.84710,0.86070
2026-02-27,0.84830,0.84980,0.84470,0.84640
2026-02-20,0.84250,0.85160,0.84180,0.84870