[26.2] Schema context support for Iceberg translation

[kbatuigas]

Description

This pull request introduces support for resolving Iceberg-enabled topic schemas within specific Schema Registry contexts, rather than always using the default context. It adds a new topic property, redpanda.schema.registry.context, and updates documentation to reflect this new capability, which is especially useful for multi-tenant or per-environment schema isolation. Related troubleshooting guidance and configuration documentation have also been updated.

Iceberg and Schema Registry context support:

• Added the redpanda.schema.registry.context topic property, allowing topics to bind Iceberg translation to a specific Schema Registry context for schema resolution.
• Updated release notes to document the new context property, describe its use cases, and link to configuration examples.
• Added troubleshooting documentation for cases where Redpanda cannot resolve a schema ID within the topic's configured Schema Registry context.

Resolves https://redpandadata.atlassian.net/browse/DOC-2192
Review deadline: 23 Jul

Preview pages

• What's New in Redpanda (updated)
• Troubleshoot Iceberg Topics (updated)
• Specify Iceberg Schema (updated)
• Schema Registry Contexts (updated)
• Topic Configuration Properties (updated)

Checks

• New feature
• Content gap
• Support Follow-up
• Small fix (typos, links, copyedits, etc)

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[micheleRP]

Docs review — suggestions

A few minor, non-blocking suggestions. Technically this looks good (and SME-approved).

1. Heading case — topic-properties.adoc

The new == Schema Registry and Validation Properties heading is Title Case, but the team standard is sentence case: == Schema Registry and validation properties ("Schema Registry" stays capitalized as a product feature).

While you're here: the headings on this page are inconsistent — "Retention and Compaction Properties", "Segment and Message Properties", and "Performance and Cluster Properties" are Title Case, while "Tiered Storage properties", "Remote Read Replica properties", and "Apache Iceberg integration properties" are sentence case. It'd be nice to bring them all to sentence case in this PR.

2. Troubleshooting bullet — iceberg-troubleshooting.adoc

The bullet says Redpanda "cannot resolve the record's schema ID within the topic's configured Schema Registry context." This is the same "not always by ID" point Matt raised on specify-iceberg-schema.adoc — in value_schema_latest mode resolution is by subject/latest, not schema ID. Consider the catch-all: "cannot resolve the record's schema within the topic's configured Schema Registry context."

3. Cloud rendering — schema-reg-contexts.adoc

The new ifdef::env-cloud[] text reads "Enabling schema_registry_enable_qualified_subjects requires a broker restart to take effect." In Redpanda Cloud, users don't restart brokers themselves — worth confirming this phrasing is accurate/actionable when this single-sourced page renders in cloud-docs.

[micheleRP]

On single-sourcing to Cloud:

The new "Resolve schemas within a Schema Registry context" section sits inside the tag=single-source region on specify-iceberg-schema.adoc, so it'll flow into the Cloud page as-is. A couple of bits are self-managed-specific — the "Starting in Redpanda 26.2… enabled by default" line, and (per the contexts page) contexts aren't exposed to users on Serverless. Do you want this section wrapped in ifndef::env-cloud[] (or given Cloud/Serverless-aware conditionals), or is that something you'll handle in the follow-up Cloud PR? No blocker either way — just checking so it doesn't render unguarded in Cloud by accident.

[micheleRP]

please see suggestions before merging!