fix(schwab): preserve bounded native account facts

[Pigbibi]

Change

• Retain optional cashBalance decimal text and securitiesAccount.type from the existing selected-account response.
• Preserve the current sizing, buying power, equity fallback, digest, requests and retry behavior.
• Bound optional decimal parsing before fixed-point formatting. Missing or invalid optional facts are omitted.
• Keep raw account-type tokens without assigning currency or interpreting broker account categories.

Validation

• 21 Schwab portfolio tests and 72 subtests passed using the existing development environment.
• Ruff, compileall and diff checks passed.
• Independent material review passed after bounding decimal formatting.

The consumer will adopt this exact head on its existing dependency baseline. This does not enable trading or establish current broker balances.