feat(gateways): Implement CreateTopics and Metadata Kafka Keys backed by the Iggy bridge

[ryerraguntla]

Summary

Wires the Kafka gateway's CreateTopics (#3538) and Metadata (#3534) handlers to the real Iggy bridge, instead of stub responses. Combined into one PR/branch because both touch IggyBridge::get_kafka_topic —
merging up front eliminates the duplicate-implementation risk a sequential rebase would have carried, rather than just deferring it.

With IGGY_KAFKA_BRIDGE_ENABLED=true:

• CreateTopics creates a real Iggy stream/topic, atomically reporting AlreadyExists instead of racing a check-then-create.
• Metadata reports real topics and partition counts pulled from the bridge.

With the bridge off (default), both remain stubs as before (CreateTopics → NOT_CONTROLLER, Metadata → every topic unknown).

Closes #3534, closes #3538.
Related to #3560

What's included

• IggyBridge::get_kafka_topic — single canonical existence/detail lookup, shared by both handlers (previously would have been two separate, differently-typed implementations).
• IggyBridge::create_kafka_topic → TopicCreationOutcome::{Created, AlreadyExists} — atomic create-or-report-exists
• IggyBridge::list_kafka_topics — enumerates override targets and default-stream topics for "list all," deduplicated.
• Kafka wire error codes tightened: ERROR_INVALID_REQUEST(42), ERROR_TOPIC_ALREADY_EXISTS(36), ERROR_INVALID_CONFIG(40), ERROR_REQUEST_TIMED_OUT(7). Removed the incorrect ERROR_INVALID_REPLICA_ASSIGNMENT(39).
• Bridge fan-out hardening, applied identically to both handlers: dedup requested topic names before any bridge call, MAX_BRIDGE_BACKED_TOPICS = 100 cap (rejected wholesale as INVALID_REQUEST, no bridge call at
  all above the cap), one wall-clock deadline over the request's aggregate bridge work (CreateTopics uses the wire timeout_ms, clamped [1s, 30s]; Metadata uses a fixed 20s, no timeout_ms field in its supported
  versions).
• Metadata: fixed v0 isAllTopics() semantics (topics == null || (topics.isEmpty() && version == 0)); added a post-bridge-call, pre-encode frame-size check (response_would_exceed_frame_size) since partition-count
  expansion (up to 1000/topic) isn't priced by the pre-decode element-count bound in bounds_guard.rs.

Test plan

• cargo build -p iggy-gateway-kafka --all-targets — clean.
• cargo fmt --all, cargo sort --no-format --workspace — clean.
• cargo clippy -p iggy-gateway-kafka --all-features --all-targets -- -D warnings — no issues.
• cargo test -p iggy-gateway-kafka — 332 passed, 16 suites (includes new create_topics_real_bridge_tests.rs, metadata_real_bridge_tests.rs, and 4 new list_kafka_topics_* cases appended to
  bridge_iggy_integration_tests.rs).
• ./scripts/ci/{trailing-whitespace,trailing-newline,license-headers,markdownlint}.sh — clean.
• git diff --check — no stray whitespace.

AI Usage

If AI tools were used, please answer:

1. Which tools? Claude code for implementation , Cursor for review and validation
2. Scope of usage? Implementation , review and validation
3. How did you verify the generated code works correctly? Test Suite and Code walkthrough
4. Can you explain every line of the code if asked? Yes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 27.59%. Comparing base (8d0e28d) to head (0a0f871).

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #4258       +/-   ##
=============================================
- Coverage     87.56%   27.59%   -59.97%     
- Complexity     1575     1576        +1     
=============================================
  Files          1284     1281        -3     
  Lines        225457   181924    -43533     
  Branches     188821   145288    -43533     
=============================================
- Hits         197416    50203   -147213     
- Misses        23314   129947   +106633     
+ Partials       4727     1774     -2953     

Components	Coverage Δ
Rust Core	12.56% <ø> (-76.20%)	⬇️
Java SDK	68.70% <ø> (+0.01%)	⬆️
C# SDK	77.41% <ø> (-0.02%)	⬇️
Python SDK	90.97% <ø> (ø)
PHP SDK	85.67% <ø> (ø)
Node SDK	96.43% <ø> (+1.68%)	⬆️
Go SDK	70.05% <ø> (ø)
see 691 files with indirect coverage changes

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[ryerraguntla]

/ready

[ryerraguntla]

/ready

[krishvishal]

I request changes for the 20 warnings below.
The largest are the 100-name Metadata cap, the batch deadlines that drop finished results, and the configs rejection that breaks Kafka Connect.

[ryerraguntla]

/ready

[ryerraguntla]

/ready

[numinnex]

@ryerraguntla resolve the conflict and from my side it's good to go.