enhance: resolve global cluster topology via DNS SRV records

[huanghaoyuanhhy]

Why

Global cluster connections previously fetched topology by hitting the global endpoint directly. This PR switches the SDK to discover ha-manager seed servers through the _grpc._tcp.<host> SRV record set, probe them concurrently, and use the topology with the highest version — with strict no-rollback semantics for every subsequent refresh path.

What changed

• SRV discovery: resolve _grpc._tcp.<endpoint hostname> to the ha-manager seed list (typically 1-5 servers, ordered; priority/weight are not differentiated in deployment). Endpoints without SRV records raise instead of falling back to the legacy direct-endpoint path.
• Seed selection & probing: probe at least two seeds concurrently when more than one is returned — uniformly at random when all weights are zero, weighted otherwise. The first fetch (no cached version) returns the first seed's answer immediately; the remaining seeds keep being polled in the background and trigger the topology replacement flow if a higher version arrives. Refreshes with a cached version poll all seeds and take the highest.
• Version management (never roll back): memory topology only advances when the fetched version is strictly higher; failures keep the cached topology. Version comparisons now run against the connection strategy's shared topology (single source of truth) instead of a private refresher snapshot, so no path can regress the version.
• Retry semantics: transient DNS failures and unreachable seeds retry with exponential backoff (3 attempts); deterministic rejections (no SRV records, server-side auth errors) fail fast without retry.
• The endpoint hostname is passed verbatim via ?endpoint=; the server derives the global cluster id from its first DNS label, so the SDK does no gcid parsing.
• Adds a dnspython dependency for SRV resolution.

Tests

• Unit tests covering SRV resolution, candidate selection, concurrent probing (first-answer fast path, background higher-version replacement, wait-all highest-version), retry/backoff, deterministic-failure fast-fail, and refresher version guarding.
• 193 passed

Deployment note

HA manager SRV records must be in place before this SDK version ships — endpoints without SRV records will raise instead of falling back to the legacy path.

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This pull-request has been approved by: huanghaoyuanhhy
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[codecov]

Codecov Report

❌ Patch coverage is 90.06211% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.10%. Comparing base (08f7e10) to head (cee8d91).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
pymilvus/client/global_topology.py	89.80%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3753      +/-   ##
==========================================
- Coverage   94.12%   94.10%   -0.02%     
==========================================
  Files          76       76              
  Lines       15821    16045     +224     
==========================================
+ Hits        14891    15099     +208     
- Misses        930      946      +16     

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