test: make DebugTelemetry span ordering deterministic

[merkulovlad]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: DebugTelemetry span ordering tests are flaky #1029

Testing Plan

The DebugTelemetry tests relied on timestamps assigned by the system clock.
Spans created in quick succession could receive equal start timestamps, causing
the stable start-time sort to preserve a nondeterministic input order.

This change assigns explicit, unique, increasing start timestamps to spans when
the tests expect a specific order. Production code and behavior are unchanged.

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Passed locally:

go test ./server/adkrest/internal/services \
  -run '^TestDebugTelemetryGetSpansBy(SessionID|EventID)$' \
  -count=100

go test ./server/adkrest/internal/services
go test ./...
golangci-lint run ./server/adkrest/internal/services/...

Manual End-to-End (E2E) Tests:

N/A. This is a test-only change with no runtime behavior changes.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.