[Galaxy Store]: Remove @Experimental annotations from Galaxy Module

[fire-at-will]

Description

Removes the @ExperimentalPreviewRevenueCatPurchasesAPI annotations from public Galaxy classes to prepare for a GA release.

Will merge + release in parallel with #3492

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Note

High Risk
High risk because it changes the public Galaxy API surface (removes @ExperimentalPreviewRevenueCatPurchasesAPI and replaces GalaxyBillingMode enum with a class-like type), which can affect source/binary compatibility and reflection-based integrations.

Overview
Prepares the Galaxy module for GA by removing @ExperimentalPreviewRevenueCatPurchasesAPI from public Galaxy-facing types and from PurchasesConfiguration.Builder.galaxyBillingMode/galaxyBillingMode property.

Updates GalaxyBillingMode from an enum to an abstract class with @JvmField singleton instances, adjusts when/conversion logic to handle non-exhaustive cases (including throwing PurchasesException for unsupported modes), and updates sample/test code and reflection usage accordingly (e.g., fetching TEST via getField).

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[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.89%. Comparing base (0857296) to head (4203e49).

Files with missing lines	Patch %	Lines
...m/revenuecat/purchases/galaxy/GalaxyBillingMode.kt	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3494      +/-   ##
==========================================
- Coverage   79.89%   79.89%   -0.01%     
==========================================
  Files         369      369              
  Lines       14871    14875       +4     
  Branches     2048     2048              
==========================================
+ Hits        11881    11884       +3     
- Misses       2157     2158       +1     
  Partials      833      833              

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[tonidero]

Nice to finally release this as stable! As mentioned, I do think it would be ideal to remove the "sealed" moving forward (we should add some linter to catch these moving forward). Lmk what you think!

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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