fix: reset tracecontext to avoid unintentional caching

[joeyzhao2018]

What does this PR do?

The customer is experiencing a case where multiple lambda invocations are put into one trace even though they have different tracecontext in their payload. This indicates that the rootTraceContext was reused/cached unintentionally.

Motivation

https://datadoghq.atlassian.net/browse/APMS-17080

Testing Guidelines

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
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[lucaspimentel]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. More of your lovely PRs please.

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[joeyzhao2018]

[HOLD] Not merging yet because this seems to be a broader issue if the fix is correct. So we should reproduce first.
Asked AI about what kind of payloads can cause this and the answer is as following

The question is about when rootTraceContext gets reused after extraction failure. Looking at the recent commit, there was
  a bug where rootTraceContext wasn't being reset between invocations, causing unintentional caching.
Based on the extraction flow, the rootTraceContext would be reused (before the recent fix) in these scenarios:

  Event payloads that would reach XRay fallback extraction:

  1. Plain object events without recognized structure - Events that don't match any of the supported event types (HTTP, SNS,
   SQS, Kinesis, EventBridge, etc.) and have no custom extractor configured.
  2. Events where all extraction methods fail - For example:
    - HTTP events with malformed or missing trace headers
    - SNS/SQS events without proper trace context in message attributes
    - Kinesis events with corrupted or missing trace data
    - Events where the Lambda context itself has no X-Ray trace information
  3. Malformed events - Events that are not objects or have unexpected structure that causes extractors to return null.

  Example event payloads that would reach XRay extraction:
  // Plain custom event with no trace headers
  { "customData": "value", "timestamp": 123456789 }

  // HTTP event with missing/invalid trace headers  
  { "headers": {}, "body": "..." }

 // SNS event with no trace context in message attributes
  { "Records": [{ "Sns": { "Message": "...", "MessageAttributes": {} } }] }

Trying these now.