refactor(core): use common arn util for parsing.

[Hweinstock]

Problem

Each component is re-implementing arn parsing, rather than relying on the top level src/core/arn.ts.

Solution

• Migrate consumers within core to this shared implementation.
• Consumers out of core (there are a few in handlers) are untouched.

Verification

• tests passing.

[agentcore-devx-automation]

Claude Security Review: no high-confidence findings. (run)

[agentcore-devx-automation]

AgentCore Harness Review

Verdict: Looks good

Pure refactor that consolidates duplicated ARN parsing onto src/core/arn.ts. I traced each call site against the code it replaces and the behavior is preserved for real inputs:

• accountIdFromArn / accountIdFromRoleArn: parseArn(arn)?.account returns the same value as arn.split(":")[4] for well-formed ARNs, and still throws for malformed input.
• roleNameFromArn → resourceNameFromArn: identical last-segment-after-/ result for IAM role ARNs (paths included).
• resourceIdFromArn in policy.tsx → resourceNameFromArn: same "strip if ARN, else pass through" contract via parseArn returning undefined for non-ARNs.
• providerFromModelArn: only invoked after startsWith("arn:"), so equivalent for foundation-model and inference-profile ARNs.
• knowledgeBaseRegion: the || → ?? swap is safe because regionFromArn already normalizes an empty region to undefined, and returns undefined for non-ARNs.
• resolveSkills in export.ts: credentialName/credentialArn are validated as simple names or ARNs, so the "contains / → last segment" fallback isn't needed in practice.

arn.ts itself is well-covered by arn.test.ts. No new user-facing behavior, so no telemetry is needed. Nothing blocking — LGTM to merge.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.73684% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.04%. Comparing base (d9ab959) to head (9d7e3eb).
⚠️ Report is 1 commits behind head on refactor.

Files with missing lines	Patch %	Lines
src/core/abTestExecutionRole.tsx	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           refactor    #2254   +/-   ##
=========================================
  Coverage     97.04%   97.04%           
=========================================
  Files           566      566           
  Lines         39409    39407    -2     
=========================================
  Hits          38244    38244           
+ Misses         1165     1163    -2     

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