refactor(data): unify vendor errors under a VendorError hierarchy

Every condition where a vendor cannot return usable data now derives from a
single VendorError base (errors.py): NoMarketDataError, VendorRateLimitError,
and VendorNotConfiguredError (still a ValueError for back-compat). Vendor-named
errors subclass the generic bases, and the router catches the base types, so a
new vendor needs no new except clause. Not-configured now has explicit
try-next-vendor handling instead of falling through the generic catch-all. The
number of error types tracks the number of distinct router reactions, not the
number of causes.

pyproject.toml

@@ -81,3 +81,8 @@ ignore = ["E501"]
 
 [tool.ruff.lint.per-file-ignores]
 "**/__init__.py" = ["F401"]   # intentional re-exports
+
+[tool.ruff.lint.isort]
+# Keep multiple aliased names from one module in a single combined import block
+# (e.g. the vendor re-exports in interface.py) instead of one statement per name.
+combine-as-imports = true