Add DeepBulkUpdateResult to give fine-grained information to the caller about failed/successful keys in batch updates

[suddendust]

Description

The current FlatPostgresCollection#bulkUpdate API silently swallows failed batches:

        } catch (Exception e) {
          LOGGER.warn(
              "Failed to update key group (size: {}): {}",
              entry.getValue().getKeys().size(),
              e.getMessage());
          // Continue with other groups - no cross-group atomicity

There's no way for callers to know which keys failed. This PR adds DeepBulkUpdateResult that callers can use to get more fine-grained info about failed docs in bulk updates.

Testing

Added UTs

Checklist:

• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules

[codecov]

Codecov Report

❌ Patch coverage is 85.18519% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.04%. Comparing base (d5c2963) to head (2d49e5c).

Files with missing lines	Patch %	Lines
...trace/core/documentstore/DeepBulkUpdateResult.java	85.71%	0 Missing and 2 partials ⚠️
...documentstore/postgres/FlatPostgresCollection.java	84.61%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #319      +/-   ##
============================================
+ Coverage     80.90%   81.04%   +0.13%     
- Complexity     1580     1615      +35     
============================================
  Files           242      243       +1     
  Lines          7632     7650      +18     
  Branches        752      754       +2     
============================================
+ Hits           6175     6200      +25     
+ Misses          968      960       -8     
- Partials        489      490       +1     

Flag	Coverage Δ
integration	81.04% <85.18%> (+0.13%)	⬆️
unit	57.12% <85.18%> (+1.80%)	⬆️

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[github-actions]

Test Results

  124 files  ±0    124 suites  ±0   35s ⏱️ -4s
  852 tests +6    851 ✅ +6  1 💤 ±0  0 ❌ ±0 
1 189 runs  +6  1 188 ✅ +6  1 💤 ±0  0 ❌ ±0 

Results for commit 2d49e5c. ± Comparison against base commit d5c2963.

♻️ This comment has been updated with latest results.