fix(firestore): preserve detailed native Firestore error messages

[Lyokone]

Description

Firestore's Android and Apple exception converters replaced detailed native SDK error messages with generic text for each normalized error code. This preserves the native message when available while retaining FlutterFire's cross-platform error codes.

Related Issues

• Fixes 🐛 [cloud_firestore] Helpful error message is lost #12045

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.

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