feat: expose parallelism_policy in fragment metadata and system catalog

[shanicky]

Introduce a new parallelism_policy field to fragment distribution metadata and propagate it through controller, meta service, and system catalog for improved visibility.

• Add parallelism_policy to FragmentDistribution in proto
• Compute and format parallelism policy for each fragment, including overrides and upstream inheritance
• Propagate new field through meta service and relevant RPCs
• Expose parallelism_policy in rw_fragments system catalog
• Enhance debugging and operational insight into fragment parallelism

dev=> SET STREAMING_USE_ARRANGEMENT_BACKFILL = false;
SET_VARIABLE
dev=> create table t(v int);
CREATE_TABLE
dev=> create materialized view m as select * from t;
CREATE_MATERIALIZED_VIEW
dev=> create materialized view m1 as select count(*) from t;
CREATE_MATERIALIZED_VIEW
dev=> select fragment_id, parallelism, parallelism_policy from rw_fragments;
 fragment_id | parallelism |   parallelism_policy
-------------+-------------+------------------------
           1 |          12 | inherit(adaptive)
           2 |          12 | inherit(adaptive)
           3 |          12 | upstream_fragment([1])
           4 |           1 | single
           5 |          12 | upstream_fragment([1])
(5 rows)

dev=> alter fragment 2 set parallelism = 3;
ALTER_FRAGMENT
dev=> select fragment_id, parallelism, parallelism_policy from rw_fragments;
 fragment_id | parallelism |   parallelism_policy
-------------+-------------+------------------------
           1 |          12 | inherit(adaptive)
           2 |           3 | override(fixed(3))
           3 |          12 | upstream_fragment([1])
           4 |           1 | single
           5 |          12 | upstream_fragment([1])
(5 rows)

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What's changed and what's your intention?

Summary
This PR enriches the fragment metadata by introducing a new field, parallelism_policy, to the streaming fragment system. The parallelism_policy field exposes the underlying logic and reasoning for each fragment’s parallelism configuration, improving transparency and debuggability across the backend, API, and frontend system catalog.

Details

• Key change 1: Adds a new optional parallelism_policy string field to the FragmentDistribution protobuf schema and propagates it to all relevant Rust structs (FragmentDesc, FragmentDistribution, RwFragment), ensuring consistent metadata exposure throughout the stack.
• Key change 2: Implements logic in the meta controller to compute and format the parallelism policy for each fragment, taking into account fragment overrides, job-level parallelism, singleton distributions, and upstream dependencies (such as NoShuffle dispatchers).
• Key change 3: Exposes the computed parallelism_policy field via the frontend system catalog table (rw_fragments), enabling users and tools to query and understand the parallelism configuration and its rationale for every fragment.

Checklist

• I have written necessary rustdoc comments.
• I have added necessary unit tests and integration tests.
• I have added test labels as necessary.
• I have added fuzzing tests or opened an issue to track them.
• My PR contains breaking changes.
• My PR changes performance-critical code, so I will run (micro) benchmarks and present the results.
• I have checked the Release Timeline and Currently Supported Versions to determine which release branches I need to cherry-pick this PR into.

Documentation

• My PR needs documentation updates.

Release note