chore(config/tracer): migrate spanTimeout

[mtoffl01]

What does this PR do?

Tracer migrates to using Config.spanTimeout instead of its own config.spanTimeout

Also, changes the default value from 0 to 10 minutes. This value is only ever read when debugAbandonedSpans is enabled, so the behavior in practice does not change.

Motivation

https://datadoghq.atlassian.net/browse/APMAPI-1748

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• New code is free of linting errors. You can check this by running ./scripts/lint.sh locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

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[pr-commenter]

Benchmarks

Benchmark execution time: 2025-12-05 18:46:46

Comparing candidate commit 7d5547f in PR branch mtoff/spantimeout with baseline commit 8eb1f41 in branch mtoff/revamp-debug-api.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 8 metrics, 0 unstable metrics.