Antalya 26.3 Backport of #101278, #102337 - Multiple Iceberg fixes

[mkmkme]

Note for reviewer

I wanted to apply only 102337 initially, but it caused conflicts due to missing changes from 101278. Therefore I decided to apply both of them at once.

Changelog category (leave one):

• Critical Bug Fix (crash, data loss, RBAC) or LOGICAL_ERROR

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Fix Logical error: 'partitions_count > 0' exception when performing consecutive ALTER TABLE UPDATE on a partitioned Iceberg table (ClickHouse#101278 by @Desel72).
Fix crash in IcebergLocal ALTER TABLE ... UPDATE when using Avro format, caused by LowCardinality/Nullable wrapper types not being unwrapped before serialization (ClickHouse#102337 by @Desel72).

Documentation entry for user-facing changes

...

CI/CD Options

Exclude tests:

• Fast test
• Integration Tests
• Stateless tests
• Stateful tests
• Performance tests
• All with ASAN
• All with TSAN
• All with MSAN
• All with UBSAN
• All with Coverage
• All with Aarch64
• All Regression
• Disable CI Cache

Regression jobs to run:

• Fast suites (mostly <1h)
• Aggregate Functions (2h)
• Alter (1.5h)
• Benchmark (30m)
• ClickHouse Keeper (1h)
• Iceberg (2h)
• LDAP (1h)
• Parquet (1.5h)
• RBAC (1.5h)
• SSL Server (1h)
• S3 (2h)
• S3 Export (2h)
• Swarms (30m)
• Tiered Storage (2h)

[github-actions]

Workflow [PR], commit [241666c]