Fix deprecated field names in MultistartOptimization result handling

[ChrisRackauckas-Claude]

Summary

Fixes deprecation warnings when using MultistartOptimization.TikTak by updating to use current OptimizationSolution field names.

Changes

Updates the _local_optimiser function in OptimizationMultistartOptimization.jl to use the current field names for optimization results:

• Changed res.minimum → res.objective
• Changed res.minimizer → res.u

Context

This resolves deprecation warnings that appeared during testing and ensures compatibility with the current OptimizationSolution API. The issue was mentioned in Discourse thread #133174.

The traits for MultistartOptimization.TikTak are already correctly set:

• requiresbounds = true (TikTak needs bounds for Sobol sequence generation)
• allowsbounds = true (TikTak supports bounds and passes them to local optimizer)

Test plan

• Ran Pkg.test("OptimizationMultistartOptimization") - all tests pass without deprecation warnings
• Verified the fix eliminates the deprecation warnings that were present before
• Formatted code with SciMLStyle

🤖 Generated with Claude Code

[ChrisRackauckas-Claude]

Additional Context

I've verified that the bounds support issue mentioned in the Discourse thread was caused by a previous bug that has already been fixed.

Testing Results:

Testing TikTak traits:
  requiresbounds: true
  allowsbounds: true
  allowscallback: false

All trait tests passed!

Testing that bounds checking doesn't throw error...
Success! Bounds checking passed - TikTak correctly supports bounds

The traits are correctly set in the current codebase:

• requiresbounds(TikTak) = true (line 7) - TikTak requires bounds for Sobol sequence generation
• allowsbounds(TikTak) = true (line 8) - TikTak supports bounds
• allowscallback(TikTak) = false (line 9) - No callback support

This PR only fixes the remaining deprecation warnings from using outdated field names.