fix(idl_gen_rust): Fix lifetime warning added in Rust 1.89

[RobJellinghaus]

Rust 1.89 added a new lifetime-related warning:
https://blog.rust-lang.org/2025/08/07/Rust-1.89.0/#mismatched-lifetime-syntaxes-lint

The Rust code generator currently emits code which trips this warning. This very small PR fixes the issue for the relevant generated functions and for the Rust flexbuffers code.

Fixes #8705

Tested as follows:

• make -j
• goldens/generate_goldens.py
• cd tests
• ./RustTest.sh

[google-cla]

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[RobJellinghaus]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

I completed the form immediately before submitting the PR, using my [email protected] email address associated with the commit. So hopefully that will propagate naturally to satisfy this check.

Edit: the check is now satisfied.