fix serverless backup resume bug

[kbrwn]

Description

This fixes duplicate relationships during zed backup when backup streams encounter retryable transport failures.

There were two related issues:

• ReadRelationships was still using the generic gRPC stream retry interceptor. On retry, that interceptor reopens the stream from the beginning, which can replay already-exported relationships and append duplicates to the backup.
• The legacy/serverless backup resume marker only stored the cursor token, not the object definition it belonged to. If a backup resumed from disk after interruption, it could restart from the wrong place and replay earlier relationships.

This PR:

• disables generic stream retries for ReadRelationships, matching the existing manual handling for bulk export/import
• stores serverless backup progress as {resource_type, cursor} so resume can restart at the correct definition
• returns a clear error when attempting to resume an old incomplete serverless backup created without resource type metadata, instead of silently duplicating data

Testing

Ran targeted tests locally:

go test ./internal/client -run TestDoesNotRetry -count=1 -v
go test ./internal/cmd -run 'TestTakeBackupRecoversFromRetryableErrors|TestTakeBackupResumesServerlessBackupAtSavedDefinition' -count=1 -v

Added coverage for:

• ReadRelationships not being retried by the generic stream retry interceptor
• serverless backup resuming from the saved definition/cursor without replaying earlier relationships
• existing manual retry handling for backup export continuing to work

Reviewers can test by:

• running the commands above
• optionally reproducing with an interrupted serverless-style backup and confirming resume does not duplicate previously exported relationships

References

Customer report describing duplicate relationships after retryable backup transport failures.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 52.17391% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.44%. Comparing base (082eee0) to head (7c07dc4).

Files with missing lines	Patch %	Lines
internal/cmd/backup.go	48.83%	14 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #644      +/-   ##
==========================================
+ Coverage   41.65%   42.44%   +0.79%     
==========================================
  Files          38       38              
  Lines        6127     6166      +39     
==========================================
+ Hits         2552     2617      +65     
+ Misses       3319     3280      -39     
- Partials      256      269      +13     

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