Propogate ens_path to plot_api StorageService.session calls

[larsevj]

Issue
Might resolve #9999

In the current code, if cls._instance is none in _base_service.py, then one guesses that the storage path is cwd, which will almost never work in proper cases.

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'just rapid-tests')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[codspeed-hq]

CodSpeed Performance Report

Merging #10036 will degrade performances by 14.05%

_{Comparing larsevj:propagate_ens_path_to_plot_api (81cc02c) with main (3f4a93e)}

Summary

❌ 1 regressions
✅ 24 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	test_direct_dark_performance_with_storage[gen_x: 20, sum_x: 20 reals: 10-summary-get_record_observations]	1.3 ms	1.6 ms	-14.05%