chore(rust): add changelog entry for wire-test environment and pagination error fixes

[iamnamananand996]

Description

Linear ticket: Refs #15856

Follow-up to #15856, which shipped two Rust SDK generator fixes without a corresponding generators/rust/sdk/changes/unreleased/ entry. This PR adds that changelog file so the next Rust SDK release picks up a patch bump and a release-notes entry describing the fixes.

The entry documents both fixes from #15856:

• Wire test generator: only emits config.environment = None; when the SDK was generated against multiple base URLs (single-URL ClientConfig has no environment field, so the previously-generated test failed to compile).
• Synchronous-paginator error-propagation tests: switched from the non-existent ApiError::Serialization variant to ApiError::Configuration so the generated pagination.rs test module compiles.

Changes Made

• generators/rust/sdk/changes/unreleased/fix-wire-test-environment-and-pagination-error.yml: new fix changelog entry covering both rust-sdk fixes from fix(rust): only clear config.environment in wire tests when multiple base URLs exist, fix pagination error tests #15856.
• Updated README.md generator (if applicable)

Testing

• Unit tests added/updated
• Manual testing completed (changelog file is data-only; the underlying generator fixes already landed in fix(rust): only clear config.environment in wire tests when multiple base URLs exist, fix pagination error tests #15856 and were validated there).

Link to Devin session: https://app.devin.ai/sessions/2e4e587dd160400390839880a5434932
Requested by: @iamnamananand996

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