Emit session_id in XE events as an integer, remove unnecessary event_source field

[azhou-datadog]

What does this PR do?

Always emit session_id as an integer, and remove unnecessary event_source field. The event_source field may be added back, but in the "sqlserver" block in the future, if we decide to make session_name's configurable. The backend is written such that it can handle session_id as an integer or as a string, to maintain backwards compatibility, see https://github.com/DataDog/dd-go/pull/188825

Tested in local dev env.
Session ID now emits as integer:

{
...
  "sqlserver": {
    "session_id": 60,
    "xe_type": "error_reported",
    "event_fire_timestamp": "2025-06-20T18:28:52.222Z",
    "primary_sql_field": "sql_text",
    "error_number": 195,
    "message": "'REPEAT' is not a recognized built-in function name."
  }
}

Motivation

Noticed mismatch of session_id types, and noticed that event_source field was in the wrong layer of the json, and I didn't need it anyhow.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[codecov]

Codecov Report

Attention: Patch coverage is 88.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 89.69%. Comparing base (5eafaa7) to head (45a528a).
Report is 4 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
activemq	?
cassandra	?
confluent_platform	?
hive	?
hivemq	?
hudi	?
ignite	?
jboss_wildfly	?
kafka	?
presto	?
solr	?
sqlserver	91.36% <88.00%> (+4.63%)	⬆️
tomcat	?
weblogic	?

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