fix: validate dump messages start message id

[proobker]

Fixes #50341

DumpMessages previously used MustUnmarshalMessageID on client-provided start_message_id bytes. A malformed message ID could panic and crash the process.

This change uses the non-panicking UnmarshalMessageID path and returns an invalid parameter error instead.

Added regression coverage for malformed Pulsar start_message_id bytes.

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[sre-ci-robot]

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[sre-ci-robot]

✅ CI Loop Results ddeb61b

Stage	Result	Duration	Tests
✅ Build	SUCCESS	14.5min	-
✅ Code-Check	SUCCESS	8.4min	-
✅ UT-GO	SUCCESS	23.5min	1012 passed
✅ UT-Integration	SUCCESS	25.7min	46 passed
✅ UT-CPP-Cov	SUCCESS	54.0min	7851 passed

Total: 82min | Pipeline | Artifacts

Overall Coverage: 71.2%
Diff Coverage: Go 100.0% (4 hit, 0 miss, 4 measurable lines, 0 unmeasured)
Diff Coverage HTML: view changed lines
Total Patch Coverage: 100.0% (4/4 measurable lines, 0 unmeasured)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.99%. Comparing base (93a76fe) to head (ddeb61b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #50349      +/-   ##
==========================================
- Coverage   79.01%   78.99%   -0.03%     
==========================================
  Files        2236     2236              
  Lines      396046   396046              
==========================================
- Hits       312925   312841      -84     
- Misses      73607    73668      +61     
- Partials     9514     9537      +23     

Components	Coverage Δ
Client	80.59% <ø> (ø)
Core	86.09% <ø> (ø)
Go	76.79% <100.00%> (-0.04%)	⬇️

Files with missing lines	Coverage Δ
internal/proxy/impl.go	71.38% <100.00%> (ø)

... and 32 files with indirect coverage changes

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