Add timing info to additional services

[0xr0bert]

Closes #3376

Submission Checklist

• Run unit tests: ./runTests.py src/test/unit
• Run cpplint: make cpplint
• Declare copyright holder and open-source license: see below

Summary

Added timing information ("Elapsed Time") to ADVI, Optimization (BFGS, L-BFGS, Newton), and Laplace Approximation Services

Intended Effect

Ensures that all major Stan services consistently report their elapsed execution time in both the console and as comments in the CSV output.

How to Verify

New test cases have been added to the following files to assert the presence of timing information:

• src/test/unit/services/optimize/bfgs_test.cpp
• src/test/unit/services/optimize/lbfgs_test.cpp
• src/test/unit/services/optimize/newton_test.cpp
• src/test/unit/services/optimize/laplace_sample_test.cpp
• src/test/unit/services/experimental/advi/fullrank_test.cpp
• src/test/unit/services/experimental/advi/meanfield_test.cpp

Side Effects

No unintended side effects.

Documentation

No external documentation updates are required as this aligns with the existing behavior of other Stan services.

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)