Make FutureResult non-Send when no-std

[Fedeparma74]

Description

This PR updates the FutureResult type alias in the async persister implementation to support non-Send futures when the std feature is disabled.

Previously, the FutureResult type always required the Send bound, which prevented no_std environments from using non-Send futures. With this change, the bound is applied only when the std feature is enabled.

This improves compatibility for embedded or no_std targets while keeping full Send safety for standard environments.

Notes to the reviewers

The change is minimal and fully backwards compatible.
It only affects compilation under no_std builds and does not modify any runtime behavior or APIs under the std feature.

Please confirm that this approach aligns with the intended async API design and does not conflict with downstream usage expectations.

Changelog notice

Changed

• Relaxed the FutureResult type alias to allow non-Send futures in no_std builds.

[coveralls]

Pull Request Test Coverage Report for Build 18875521602

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 84.995%

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Totals
Change from base Build 18850737341:	0.0%
Covered Lines:	7001
Relevant Lines:	8237

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💛 - Coveralls