Derive the test symbol from the registry instead of naming VIX

[mspinola]

main is red at 80b60df. This fixes it.

What happened

#18 and #19 are each correct, and each was green on its own base. They break when combined:

• Refuse an unregistered --symbols instead of reporting wrote=0 #18 added tests/test_cli_symbols.py, which used VIX as its example of a symbol that is in the registry.
• Drop the four CBOE implied-vol indices from the registry #19 retired VIX from registry.yaml.

#18 branched before #19 landed, so the two never ran against each other. The merge produced a commit no pipeline had tested, and two cases went red:

assert update.main(["--bars", "--domain", "equities", "--symbols", "VIX"]) == 0
E   SystemExit: 2
marketdata-update: error: not in the marketdata registry: VIX.

What did not go wrong

The guard did exactly what #18 built it to do — refuse --symbols VIX against a registry that no longer carries VIX — and #19's own commit message predicted that consequence in writing. The production code is right in both PRs. Only the test fixture asserted a world that no longer exists.

The fix

The fixture reads an equities symbol out of the registry instead of naming one:

def a_registered_equities_symbol() -> str:
    for s in all_symbols():
        if s.domain == "equities" and s.yahoo:
            return s.internal
    raise RuntimeError("registry carries no equities symbol to test with")

There is no longer a symbol name to keep in step, so a future retirement cannot turn these tests red for a reason unrelated to what they assert. That is the same class of drift the module was written to catch, so it should not have been reproduced in its own fixture.

Verification

ruff check src tests clean. pytest tests/ -q -m "not network" — CI's exact invocation — 205 passed, 12 deselected.

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