feat: auto-chunk oversized FileUpdate contents via ExecuteAll

[Dosik13]

Description:

Add opt-in auto-chunking to FileUpdateTransaction, mirroring FileAppendTransaction, so files larger than a single transaction can be updated without hand-writing the FileUpdate + FileAppend two-step. SetContents previously sent one transaction, so contents over the ~6 KiB single-transaction limit were rejected with TRANSACTION_OVERSIZE.

• Add FileUpdateTransaction.ExecuteAll, which overwrites the file with the first chunk and appends the remainder, so contents over the single-transaction limit can be updated in one call; it returns the FileUpdate response followed by one response per append chunk
• Add SetMaxChunks/GetMaxChunks and SetMaxChunkSize/GetMaxChunkSize (defaults 20 / 2048, matching FileAppendTransaction)
• Guard Schedule() so chunked contents (more than one chunk) return ErrMaxChunksExceeded, mirroring FileAppendTransaction.Schedule
• Add errFileUpdateChunkingRequiresFileID / errFileUpdateChunkingRequiresUnfrozen sentinels for the chunking preconditions
• Keep Execute single-transaction semantics unchanged (still returns TRANSACTION_OVERSIZE for oversized contents)
• Extend the existing file_update_transaction_unit_test.go / _e2e_test.go with chunking tests (chunk-count math, boundary, guards, scheduling; >6 KiB round-trip verified via FileContentsQuery and FileInfoQuery, plus a single-chunk case), and add an examples/file_update_chunked example

Related issue(s):

Fixes #

Notes for reviewer:

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.)

[lfdt-bot]

✅ Snyk checks have passed. No issues have been found so far.

Status	Scan Engine	Critical	High	Medium	Low	Total (0)
✅	Open Source Security	0	0	0	0	0 issues

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[codecov]

Codecov Report

❌ Patch coverage is 81.53846% with 12 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
sdk/file_update_transaction.go	81.53%	6 Missing and 6 partials ⚠️

Files with missing lines	Coverage Δ
sdk/errors.go	65.51% <ø> (ø)
sdk/file_update_transaction.go	89.75% <81.53%> (-5.34%)	⬇️

... and 3 files with indirect coverage changes

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