fix(search): expose LastPullTimestamp and PushedBy on index ImageSummary

[cainydev]

ImageIndex2ImageSummary was missing LastPullTimestamp assignment, causing multi-arch image queries to always return null for this field. Also adds the PushedBy field (already stored in MetaDB) to the GraphQL schema and both conversion paths (manifest and index).

What type of PR is this?

bug, feature

Which issue does this PR fix:

Related to #3859 — PushedBy is now queryable via GraphQL as an alternative to webhook event metadata.

What does this PR do / Why do we need it:

1. Bug fix: ImageIndex2ImageSummary never assigned LastPullTimestamp to the output ImageSummary, so multi-arch image queries always returned null for this field — even though the single-manifest path (ImageManifest2ImageSummary) set it correctly.
2. Feature: PushedBy (the user who pushed the image) is stored in all MetaDB backends (BoltDB, DynamoDB, Redis) with a proto definition, but was completely absent from the GraphQL API. This adds it to the ImageSummary schema type and both conversion paths.

If an issue # is not available please add repro steps and logs showing the issue:

Query LastPullTimestamp on any multi-arch image via GraphQL — it always returns null regardless of pull history.

Testing done on this change:

• Extended TestTaggedTimestamp with LastPullTimestamp and PushedBy assertions for both manifest and index image summaries
• All 302 assertions in convert package pass
• Full binary builds successfully

Automation added to e2e:

No — unit tests only. The fix is in conversion logic, not API transport.

Will this break upgrades or downgrades?

No. PushedBy is a new optional GraphQL field (nullable String). Existing queries are unaffected.

Does this PR introduce any user-facing change?:

GraphQL: fix `LastPullTimestamp` always returning null for multi-arch images; add `PushedBy` field to `ImageSummary`

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.47%. Comparing base (2ba0525) to head (e96121e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3865   +/-   ##
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  Coverage   91.47%   91.47%           
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  Files         194      194           
  Lines       27525    27531    +6     
=======================================
+ Hits        25179    25185    +6     
  Misses       1520     1520           
  Partials      826      826           

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