fix: resolve SessionWatcher goroutine leak and unstable UT in querycoordv2

[congqixia]

Related to #44620 Related to unstable ut "internal/querycoordv2 TestServer/TestNodeUp"

Introduce SessionWatcher interface to fix race condition and goroutine leak that caused unstable unit test TestServer/TestNodeUp.

Changes:

• Add SessionWatcher interface with EventChannel() and Stop() methods
• Refactor WatchServices() to return SessionWatcher instead of raw channel
• Fix cleanup order in QueryCoordV2: stop watcher before session
• Update DataCoord, ConnectionManager to use SessionWatcher
• Add MockSessionWatcher for testing

Fixes race condition between session context cancellation and internal loop exit. Eliminates goroutine leak by providing explicit lifecycle management.

[sre-ci-robot]

[ci-v2-notice] Notice: We are gradually rolling out the new ci-v2 system.

• Legacy CI jobs remain unaffected, you can just ignore ci-v2 if you don't want to run it.
• Additional "ci-v2/*" checkers will run for this PR to ensure the new ci-v2 system is working as expected.
• For tests that exist in both v1 and v2, passing in either system is considered PASS.

To rerun ci-v2 checks, comment with:

• /ci-rerun-code-check // for ci-v2/code-check
• /ci-rerun-build // for ci-v2/build
• /ci-rerun-ut-integration // for ci-v2/ut-integration
• /ci-rerun-ut-go // for ci-v2/ut-go
• /ci-rerun-ut-cpp // for ci-v2/ut-cpp
• /ci-rerun-ut // for all ci-v2/ut-integration, ci-v2/ut-go, ci-v2/ut-cpp
• /ci-rerun-e2e-arm // for ci-v2/e2e-arm

If you have any questions or requests, please contact @zhikunyao.

[congqixia]

/ci-rerun-ut-go

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@congqixia cpu-e2e job failed, comment /run-cpu-e2e can trigger the job again.

[congqixia]

/run-cpu-e2e

[codecov[bot]]

Codecov Report

:x: Patch coverage is 95.65217% with 2 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 76.50%. Comparing base (caed0fe) to head (f6a9728). :warning: Report is 31 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd/tools/migration/migration/runner.go	0.00%	2 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #45627       +/-   ##
===========================================
- Coverage   83.18%   76.50%    -6.68%     
===========================================
  Files         521     1875     +1354     
  Lines       81313   292178   +210865     
===========================================
+ Hits        67642   223539   +155897     
- Misses      13671    61240    +47569     
- Partials        0     7399     +7399     

Components	Coverage Δ
Client	78.17% <ø> (∅)
Core	83.19% <98.38%> (+0.01%)	:arrow_up:
Go	74.62% <95.52%> (∅)

Files with missing lines	Coverage Δ
internal/datacoord/server.go	68.00% <100.00%> (ø)
internal/distributed/connection_manager.go	71.27% <100.00%> (ø)
internal/querycoordv2/server.go	76.03% <100.00%> (ø)
internal/util/sessionutil/session_util.go	75.59% <100.00%> (ø)
cmd/tools/migration/migration/runner.go	0.00% <0.00%> (ø)

... and 1349 files with indirect coverage changes

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[sre-ci-robot]

[ci-v2-notice] Notice: We are gradually rolling out the new ci-v2 system.

• Legacy CI jobs remain unaffected, you can just ignore ci-v2 if you don't want to run it.
• Additional "ci-v2/*" checkers will run for this PR to ensure the new ci-v2 system is working as expected.
• For tests that exist in both v1 and v2, passing in either system is considered PASS.

To rerun ci-v2 checks, comment with:

• /ci-rerun-code-check // for ci-v2/code-check
• /ci-rerun-build // for ci-v2/build
• /ci-rerun-ut-integration // for ci-v2/ut-integration
• /ci-rerun-ut-go // for ci-v2/ut-go
• /ci-rerun-ut-cpp // for ci-v2/ut-cpp
• /ci-rerun-ut // for all ci-v2/ut-integration, ci-v2/ut-go, ci-v2/ut-cpp
• /ci-rerun-e2e-arm // for ci-v2/e2e-arm

If you have any questions or requests, please contact @zhikunyao.

[congqixia]

/ci-rerun-ut-integration

[mergify[bot]]

@congqixia cpu-e2e job failed, comment /run-cpu-e2e can trigger the job again.

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/run-cpu-e2e

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/ci-rerun-ut-integration

[congqixia]

/ci-rerun-ut-integration

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: congqixia, liliu-z

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