feat(datadog_llm_obs): cost tag dimensions, router decision fields, reasoning token metric, redaction gating (#39402)

* feat(datadog_llm_obs): cost tag dimensions, router decision fields, reasoning token metric, redaction gating

* test(datadog_llm_obs): satisfy test quality gate

* fix: forward integer parent_id as its string form

* fix(datadog): sanitize redacted message roles

* fix(datadog): keep the A2A agent role on redacted spans

* fix(datadog): merge current staging budget

* style(datadog): format redaction tests

* fix(datadog): handle malformed redacted roles

* test(datadog): put the test quality suppression on the reported line

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

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Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>