grpc: terminate subscriptions with UNAVAILABLE on broker shutdown

[harvinderRathore]

Problem
On broker shutdown, active streaming subscriptions previously completed with OK,
which prevents clients from detecting outage and retrying properly.

Fix
Wrap subscribe streams with a tokio::select! that also listens to the broker's
shutdown broadcast. When triggered, terminate the stream with
Status::unavailable("Databroker shutting down").

Testing

• Manual: started broker (--insecure), subscribed via CLI, published values,
  sent SIGINT to broker. CLI shows “Server gone, Subscription stopped”.
• Build/tests: cargo build, cargo test pass locally.

Linked issue
Fixes #125

Signed-off-by: Harvinder Singh Rathor harvinderrathor9@gmail.com

[codecov]

Codecov Report

❌ Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.08%. Comparing base (9b4c5be) to head (af7f1b0).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
databroker/src/grpc/kuksa_val_v1/val.rs	0.00%	5 Missing ⚠️
databroker/src/grpc/sdv_databroker_v1/broker.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #171      +/-   ##
==========================================
- Coverage   62.10%   62.08%   -0.03%     
==========================================
  Files          37       37              
  Lines       17088    17094       +6     
==========================================
  Hits        10612    10612              
- Misses       6476     6482       +6     

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[SebastianSchildt]

Thank you for the contribution.

There are some minor issues with linitng (we are using cargo fmt and clippy). The pre-commit hoo can help with that, see the notes here https://github.com/eclipse-kuksa/kuksa-databroker/blob/main/CONTRIBUTING.md#pre-commit-set-up

Also, we require that commits are signed-off and that you "signed" the Eclipse Contributor Agreement, whereas signing means

• making a free Eclipse account https://accounts.eclipse.org/user/register?destination=/user/login
• In your Eclipse Account link your gihub identiy
• "sign" the ECA, which basically means setting a checkmark and typing your name in the Eclipse account. Not much legalese there, you basically only confirm you contribute to an Eclipse project you are abiding by the license (Apache in our case) and that the work you provided is your own.

In case you get stuck, just write.

So mich for the formal stuff, wrt to "tech" content @rafaeling do you think these change makes sense/does not break anything?

[harvinderRathore]

@SebastianSchildt & @rafaeling
This update applies rustfmt formatting in val.rs to resolve the cargo fmt pre-commit failure.
No functional changes were made.
Requesting review and workflow approval.

[harvinderRathore]

@SebastianSchildt & @rafaeling
CI is passed. Verified on my repo. Ref the image
No functional changes were made.
Requesting review and workflow approval.

[rafaeling]

Looks good to me and @lukasmittag, the PR doesn't introduces any break changes to kuksa.val.v2.
However, UNAVAILABLE error code is used differently in kuksa.val.v2, we would have to re-check again in the future how to handle it correctly
Bug created: #173