RUM-2780: Return no-op core when instance is already initialized

[ambushwork]

What does this PR do?

we should return a no-op instance and prevent leaks between orgs.

Motivation

RUM-2780

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.42%. Comparing base (d9dbeca) to head (1efebab).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3271      +/-   ##
===========================================
+ Coverage    71.38%   71.42%   +0.04%     
===========================================
  Files          942      942              
  Lines        34823    34822       -1     
  Branches      5901     5901              
===========================================
+ Hits         24857    24871      +14     
+ Misses        8297     8283      -14     
+ Partials      1669     1668       -1     

Files with missing lines	Coverage Δ
...ore/src/main/kotlin/com/datadog/android/Datadog.kt	71.43% <100.00%> (-0.31%)	⬇️

... and 35 files with indirect coverage changes

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Return existing core on duplicate initialization

Returning NoOpInternalSdkCore here breaks initialize() idempotency for callers that keep and reuse the returned SdkCore (for example, val core = Datadog.initialize(...); Rum.enable(..., core)). In a common defensive pattern where a library calls initialize() after the app already did, the second call now hands back a no-op core, so subsequent feature registration and user/context updates are silently dropped even though a real instance is already active.

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[hamorillo]

🚀 LGTM! As you mentioned in the ticket, we need to block merging this PR until v4, though.

[hamorillo]

nitpick: Should we add that we can now return NoOpInternalSdkCore?