Add single-item convenience wrappers for bulk partial-success APIs

[fredvisser]

Summary

Several SystemLink client APIs only expose bulk operations that return partial-success envelopes (items + failed + error). When callers need to create or update a single item — by far the most common case — they must wrap the call in a list, then manually unpack the response and check for partial failures. This PR eliminates that boilerplate by adding singular convenience methods to six client classes.

What changed

Client	New methods
TestMonitorClient	create_result, create_step, update_result, update_step
ProductClient	create_product, update_product
SpecClient	create_spec, update_spec
TestPlanClient	create_test_plan, update_test_plan, schedule_test_plan, create_test_plan_template
WorkItemClient	create_work_item, update_work_item, schedule_work_item, create_work_item_template, update_work_item_template
AssetManagementClient	create_asset

A shared helper unwrap_single_item_partial_success() (in nisystemlink/clients/core/helpers/_partial_success.py) centralizes the unwrap-or-raise logic so each wrapper stays minimal.

Why this is beneficial

1. Reduces caller boilerplate — common single-item operations go from ~6 lines to 1.
2. Safer error handling — partial-success failures are translated into ApiException with full error context, rather than silently returning empty lists.
3. Consistent pattern — all wrappers follow the same contract (raise on failure, return typed item on success), making the API surface predictable.
4. Non-breaking — the existing bulk methods remain unchanged; the new methods are purely additive.

Testing

• 24 new unit tests covering the shared helper and all 18 convenience methods.
• Full CI validation passes: poe check (formatting), poe lint, poe types (mypy, 379 source files), poe test (410 tests passed).