[fix][broker] Decode topic name in the scalable topic migrate endpoint - #26711

Motivation

The scalable-topic migrate endpoint (POST .../persistent/{tenant}/{namespace}/{topic}/migrateToScalable) declares its {topic} path segment @Encoded but spliced it straight into the derived topic identities without URL-decoding — unlike its sibling createScalableTopic, which calls Codec.decode first. For any local name containing a character the admin client percent-encodes in the REST path (a space, a non-ASCII character, …), every identity the endpoint derived was wrong:

• Migrating a b checked existence of persistent://t/ns/a%20b, so the request 404s ("Topic does not exist") even though the topic exists.
• If a topic literally named like the encoded spelling exists, the endpoint would authorize against, existence-check, and terminate that other topic, and write the scalable record under topic://t/ns/a%20b — an identity no v5 client lookup ever derives (clients resolve the scalable identity from the raw local name via TopicName#toScalableTopic), so the migrated record is invisible to clients and to the PIP-475 recreate guard.

Modifications

Decode the @Encoded segment with Codec.decode before building the topic names, exactly as createScalableTopic already does. Added ScalableTopicsMigrateTest, which drives the endpoint with a mocked pipeline and asserts the authorization target, the metadata-lookup name, and the existence-check name all use the decoded local name (a%20b → a b, caf%C3%A9 → café).

This is the migrate write-path; the read-side endpoints in the same resource share the same missing-decode and would be coherent follow-ups.

Verifying this change

Covered by the added unit test ScalableTopicsMigrateTest. It fails without the fix (expected [persistent://tenant/ns/a b] but found [persistent://tenant/ns/a%20b]) and passes with it.

Found by code inspection while working on the related TopicName#toScalableTopic identity fix (#26668).

Does this pull request potentially affect one of the following parts:

• Dependencies: no
• The public API: no
• The schema: no
• The default values of configurations: no
• The threading model: no
• The binary protocol: no
• The REST endpoints: behavior fix only (the migrate endpoint now resolves encoded topic names correctly)
• The admin CLI options: no
• The metrics: no
• Anything that affects deployment: no

Documentation

• doc-not-needed