Standardize UpstreamGrpcSender shutdown to await channel termination

[thswlsqls]

Related to #8280

Description

• shutdown() called channel.shutdownNow() and returned ofSuccess() right away, signalling completion before the channel had terminated.
• It now awaits termination for up to 5 seconds on a daemon thread, logs a warning on timeout, and succeeds either way — mirroring OkHttpHttpSender, standardized in Standardize OkHttpHttpSender shutdown to await executor termination #8495.
• Only the fallback channel we create is shut down; a caller-supplied channel is untouched and still returns immediately.
• The okhttp senders also run cancelAll()/evictAll() unconditionally, but ManagedChannel has no API to cancel in-flight RPCs without closing the channel, so there is no counterpart here.
• The executor field is left alone: it comes from ExtendedGrpcSenderConfig.getExecutorService() and is caller-owned.
• JdkHttpSender remains in Standardize sender shutdown implementations #8280, so this PR does not close it.

Testing done

• Added UpstreamGrpcSenderTest: shutdown_ManagedChannel_WaitsForChannelTermination, shutdown_UnmanagedChannel_ReturnsImmediatelyAndLeavesChannelOpen, shutdown_ChannelDoesNotTerminateInTime_Succeeds. A fake ManagedChannel controls termination timing. The first fails against the previous implementation.
• ./gradlew :exporters:sender:grpc-managed-channel:check — 3 tests passed. The module had no test source set before; no build script change needed.
• No apidiff or CHANGELOG entry: internal package, signature unchanged, matching Standardize OkHttpHttpSender shutdown to await executor termination #8495.

[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.60%. Comparing base (b86f040) to head (300b1f6).

Files with missing lines	Patch %	Lines
...pc/managedchannel/internal/UpstreamGrpcSender.java	84.61%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8624      +/-   ##
============================================
- Coverage     91.60%   91.60%   -0.01%     
- Complexity    10343    10345       +2     
============================================
  Files          1013     1013              
  Lines         27352    27365      +13     
  Branches       3215     3216       +1     
============================================
+ Hits          25057    25068      +11     
- Misses         1567     1569       +2     
  Partials        728      728              

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