sdks/ts: fix serializer bugs and agentError null crash

[Myestery]

Summary

• Fix missingObjectKey argument swap: arguments were reversed (tsValue, 'tag') → ('tag', tsValue), producing wrong error messages
• Fix handleObjectMatch null check: was comparing to string literal 'interface' instead of checking for null
• Fix u64 BigInt conversion: properly convert number to BigInt instead of casting as any
• Add null guards in serializeTsValueToBinaryReference and serializeTsValueToTextReference (typeof null === 'object' was passing the guard)
• Fix isAgentError crash on null input: add null/non-object guard before accessing properties

Context

Split from #2845 — this PR contains only the bug fixes, no lint rule changes or type refactoring.

Test plan

• Existing SDK tests pass
• Manual verification that isAgentError(null) returns false instead of crashing
• Serializer correctly handles missingObjectKey error messages

[github-actions]

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I have read the CLA Document and I hereby sign the CLA

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1 out of 2 committers have signed the CLA.
✅ (vigoo)[https://github.com/vigoo]
❌ @Myestery
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