fix(ios): send one _makePurchase message per purchase attempt

[vegaro]

What changed

purchaseProduct and purchasePackage sent the response to _makePurchase twice on failure — once inside the error branch, and again from the unconditional send after the if/else:

if (error) {
    ...
    [self sendJSONObject:response toMethod:MAKE_PURCHASE];   // first
} else {
    ...
}
[self sendJSONObject:response toMethod:MAKE_PURCHASE];       // second, always runs

Only the first delivery reaches the caller today, because _makePurchase clears MakePurchaseCallback before invoking it, so the duplicate is dropped by the null guard. The extra UnitySendMessage is still wasted work on the wrong contract. The two win-back offer handlers already had the correct single-send shape; this makes all four consistent.

Also in this PR

response[@"userCancelled"] = false appeared in three places. false is 0, so it boxes to nil, and assigning nil through an NSMutableDictionary subscript removes the key rather than storing it — the success payload shipped with no userCancelled field at all. Unity reads the missing key as false via SimpleJSON, so the behaviour was correct by accident. Replaced with @NO so the payload matches what PurchaseResult expects. purchasePackage already used @NO, which is what made the inconsistency visible.

Overlaps with #1008

#1008 rewrites these same lines for request-ID correlation and carries the double send unchanged. It will conflict with this PR, and the duplicate matters more there: once callbacks are consumed by request ID, the second message arrives as an unrecognised request rather than being dropped by a null check.

Verification

• Compiles: archive-ios, archive-ios-cocoapods, build-integration-tests-ios and build-subtester-ios all pass on CI, so the plugin builds and links.
• Runtime behaviour of a failed purchase — not covered.

No test in this repo exercises PurchasesUnityHelper.m at runtime, and Edit Mode tests do not compile the iOS plugin, so the single-send behaviour on a failed purchase is unverified. Worth a manual check in Subtester (cancel a purchase, confirm one _makePurchase log line) before this leaves draft.

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[RevenueCat-Danger-Bot]

	1 Error
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Label	Description
pr:feat	A new feature. Use along with pr:breaking to force a major release.
pr:fix	A bug fix. Use along with pr:force_minor to force a minor release.
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pr:next_release	Preparing a new release
pr:dependencies	Updating a dependency
pr:phc_dependencies	Updating purchases-hybrid-common dependency
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[vegaro]

closing in favor of #1042