Add gold and silver commodity endpoints

[Jackisome]

Summary

Adds wrappers for the official Alpha Vantage gold/silver endpoints in Commodities, aligned with the existing commodity method style (get_wti(), get_copper(), etc.):

Historical series:

• get_gold(interval="monthly", symbol="GOLD") -> GOLD_SILVER_HISTORY
• get_silver(interval="monthly", symbol="SILVER") -> GOLD_SILVER_HISTORY

Realtime spot prices:

• get_gold_spot(symbol="GOLD") -> GOLD_SILVER_SPOT
• get_silver_spot(symbol="SILVER") -> GOLD_SILVER_SPOT

The official docs accept:

• Gold: GOLD / XAU
• Silver: SILVER / XAG

Historical intervals are documented as:

• daily
• weekly
• monthly

Relationship to #387

This PR overlaps with #387 on the historical endpoint (GOLD_SILVER_HISTORY). I updated this PR to follow the same package-style API direction as #387 (get_gold() / get_silver()), while also adding:

• realtime spot endpoint support (get_gold_spot() / get_silver_spot())
• test coverage for history and spot methods
• XAU / XAG symbol coverage in tests

If maintainers prefer #387 as the base implementation, I am happy to adapt this PR to contribute only the missing spot endpoint support and tests.

Context

This follows up on the previously closed #385. The duplicate follow-up issue #389 was closed in favor of this implementation PR.

The official API supports these endpoints, but they were not exposed through the Python wrapper. Using ForeignExchange / FX_DAILY is not a substitute for XAG/USD or XAU/USD, because gold/silver are handled by the dedicated GOLD_SILVER_* endpoints.

Tests

python -m pytest test_alpha_vantage/test_commodities.py -q

Result:

4 passed

I also tried a broader existing test file, but it appears blocked by an unrelated existing import problem around sectorperformance.py not being present in the branch.

[Jackisome]

I noticed that #387 already implements the historical gold/silver endpoint via get_gold() and get_silver().

I have updated this PR to align with that package API style instead of the earlier get_gold_silver_* naming:

• get_gold(...) / get_silver(...) for GOLD_SILVER_HISTORY
• get_gold_spot(...) / get_silver_spot(...) for GOLD_SILVER_SPOT

So there is still overlap with #387 on the historical endpoint, but this PR additionally covers the realtime spot endpoint and adds tests for both history and spot behavior, including XAU / XAG aliases.

If maintainers prefer #387 as the base, I am happy to adjust this PR further to contribute only the spot support and tests.