feat(connectors): add Apache Fluss source connector

[seokjin0414]

What

Adds a source connector that reads an Apache Fluss
log table and publishes each row into an Apache Iggy stream as JSON. This is
phase 1 of #3688: log tables and Schema::Json.

How it works

The connector resolves the table schema while opening, so a missing table, a
primary-key table, a partitioned table or a column type with no JSON form all
fail at startup rather than once per batch. The log scanner is created once in
open() and reused, which works because it owns its client state rather than
borrowing the connection.

Offsets are tracked per bucket and returned in every ProducedMessages,
including empty polls, so the runtime can persist them. On restart, buckets
already present in the state keep their offset and only new ones fall back to
starting_offset, which keeps a widened bucket count from rewinding buckets
that were already consumed. Each message carries an id derived from its bucket
and offset so Apache Iggy can deduplicate after an at-least-once replay.

Column projection is pushed down to the server when columns is set. Temporal
values keep their Fluss-native integer form, because Fluss carries no timezone
that a formatted string could honour.

The packaging question I asked in #3688

fluss-rs compiles its protocol definitions in a build script, so the workspace
now needs a system protoc. I raised this in
#3688 (comment) and said I
leaned towards registering the crate as a regular workspace member, so that is
what the last two commits do: protobuf-compiler goes into the shared Rust
setup action, which every Rust workflow already uses, and CONTRIBUTING gets the
prerequisite.

If you would rather keep it out of the workspace build via exclude, only
build: register the Apache Fluss source in the workspace and
ci: install protoc ... need reworking. The connector and its test do not
change. Happy to flip it.

Testing

Unit tests cover config validation, the state round-trip, offset resolution and
the row-to-JSON mapping (including nulls, base64 for binary and non-finite
floats).

The integration test appends rows to a real Fluss log table and asserts on what
comes back out of the Apache Iggy topic, offsets and bucket included. The
cluster runs as a single container: the image ships local-cluster.sh, which
starts the same embedded ZooKeeper, coordinator and tablet server, but it
rewrites the tablet server's bind port to 0 so it cannot collide with the
coordinator, and a random port inside the container cannot be published to the
host. Giving the tablet server an explicit second port keeps both reachable and
lets both advertise localhost, which then resolves the same way inside the
container and from the test process.

Notes on scope

Two limits come from the released fluss-rs 0.1.0 rather than from the
connector, and each is rejected at startup with the reason rather than silently
ignored:

• Primary-key changelog scanning is not in the release, so table_type = "primary_key" is refused.
• payload_format = "arrow_ipc" is doable (0.1.0 does expose an Arrow
  RecordBatch scanner) but it is a separate scanner with its own
  offset-tracking path, so I left it for a follow-up.

starting_offset = "latest" is supported: buckets without persisted state get
their tail offset from FlussAdmin::list_offsets(.., OffsetSpec::Latest) at
open time (verified against a real 0.9.1 cluster: an empty bucket reports 0 and
the offset moves to the row count after appends, so subscribing there yields
only new records).

Refs #3688

[github-actions]

Thanks for the PR. It is labeled S-waiting-on-review and queued for review.

Slash commands (own line, regular comment) move it around the queue:

• /ready - back to S-waiting-on-review after addressing feedback
• /author - flip to S-waiting-on-author while you finish changes
• /request-review @user-or-team - request a reviewer

See CONTRIBUTING.md for details.

[codecov]

Codecov Report

❌ Patch coverage is 79.87013% with 93 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.31%. Comparing base (102484a) to head (26bf766).
⚠️ Report is 25 commits behind head on master.

Files with missing lines	Patch %	Lines
core/connectors/sources/fluss_source/src/lib.rs	83.74%	46 Missing and 7 partials ⚠️
...ore/connectors/sources/fluss_source/src/mapping.rs	70.58%	32 Missing and 8 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3799      +/-   ##
============================================
+ Coverage     75.96%   76.31%   +0.35%     
- Complexity      969     1046      +77     
============================================
  Files          1320     1335      +15     
  Lines        157608   165084    +7476     
  Branches     130990   137638    +6648     
============================================
+ Hits         119720   125990    +6270     
- Misses        34297    35279     +982     
- Partials       3591     3815     +224     

Components	Coverage Δ
Rust Core	76.34% <79.87%> (+0.37%)	⬆️
Java SDK	63.67% <ø> (+0.95%)	⬆️
C# SDK	71.13% <ø> (-1.13%)	⬇️
Python SDK	88.93% <ø> (-4.17%)	⬇️
PHP SDK	84.52% <ø> (ø)
Node SDK	96.34% <ø> (+1.11%)	⬆️
Go SDK	43.08% <ø> (ø)

Files with missing lines	Coverage Δ
...ore/connectors/sources/fluss_source/src/mapping.rs	70.58% <70.58%> (ø)
core/connectors/sources/fluss_source/src/lib.rs	83.74% <83.74%> (ø)

... and 178 files with indirect coverage changes

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