Fix/ep cart qty remove errors

[mzaintariq]

What does this MR do?

Exposes designer-facing cart drawer mutation errors on $ctx.quantityControl.error and $ctx.removeItemState.error, aligned with the post-#405 proxy stable code architecture (same shopper copy mapping as ATC). Designers can bind these in Studio the same way as $ctx.addToCartState.error.

Changes

• EPCartItemQuantityControl: track mutation error on DataProvider quantityControl; clear on new attempt / item change; map via shared helper; previewState: "error" sample copy
• EPCartItemRemoveButton: same pattern for removeItemState
• New cartMutationErrorCopy helper (shared with ATC) mapping insufficient_stock / no_session → shopper copy; unknown codes (incl. dispatch_failed) → generic fallback
• Refactor ATC to use the shared helper (behavior unchanged)
• COMPONENTS.md + cart drawer / helper unit tests

Design decisions

• Branch on proxy code, not message text — production proxy withholds messages
• Keep qty stock-cap / rollback / in-flight behavior from fix(ep-commerce): cart mutation errors and multilocation quantity updates #400/fix(ep-commerce): reject no-session cart mutations and carry error codes #405; this MR only adds designer-visible error state
• Shared copy helper avoids drift between ATC, qty, and remove

Testing

• Unit tests: quantityControl.error / removeItemState.error lifecycle, preview sample, insufficient_stock / no_session / dispatch_failed mapping; cart-mutation-error-copy.test.ts
• Manual (Studio): bound error text under qty control and remove button; verified mapped stock/session errors after exercising mutations (local host; stock + guard temporarily disabled for multilocation max-stock cases — not included in this MR)

Reviewer notes

• Scope is cart-drawer error exposure + shared copy helper only