Fix docs: correct Workspace error-throwing behavior for commands and fingerprints

[Copilot]

Two documentation sections incorrectly described Workspace as silently ignoring invalid inputs when it actually throws.

Changes

• docs/implementing-a-diagnostic-provider.md: Corrected the description of this.commands — Workspace throws when a command not declared in that set is executed, it does not ignore it.
• docs/implementing-a-dismissal-store.md: Updated the addDismissal contract to reflect that Workspace.dismissDiagnostic() throws on null/undefined fingerprints; callers must only pass dismissible diagnostics and store implementations should not expect to receive null/undefined fingerprints via Workspace.

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Latest commit: 0609eff

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