Default report frequencies and reject values below 100ms

[driv3r]

Fixes #214. Supersedes #215.

Problem

ProgressReportFrequency and StateReportFrequency have no default and no validation. The reporting loops wait on time.After(time.Duration(freq) * time.Millisecond), so with a callback URI set and the frequency left at 0 (or set negative) the loop fires immediately, forever. Each iteration builds a full Progress (or serializes the whole state) and POSTs it synchronously.

Smoke run with the real ghostferry-copydb binary on a 3-row table, ProgressCallback set and no frequency:

binary	config	progress POSTs	result
main	frequency unset	1053 in 1.5 s	completes
this PR	frequency unset	2 (cutover + done)	completes
this PR	ProgressReportFrequency: 500	4	completes
this PR	ProgressReportFrequency: -1	0	failed to validate config: ProgressReportFrequency must be at least 100 milliseconds, got -1

Fix

In Config.ValidateConfig:

• 0 defaults to 60000 ms, matching how other optional numeric settings are defaulted (BinlogEventBatchSize, DBReadRetries, ...).
• Values below 100 ms are rejected when the matching callback URI is set. That covers negative values and "meant seconds, wrote milliseconds" typos.

#215 clamped to a 1-minute minimum inside the loops instead. That would silently override the 500 ms the integration tests use and the 1500 ms the benchmark uses. Rejecting at validation reports the misconfiguration at startup instead.

All in-repo entry points (copydb, sharding, integration ferry, test helpers) call ValidateConfig.

Repro

The first commit adds only the tests:

• TestConfig/TestReportFrequenciesDefaultWhenCallbacksAreSet: before the fix the frequency stayed 0.
• TestConfig/TestReportFrequenciesBelowMinimumAreRejected: before the fix there was no error for -1 or 50.

Verification (local, MySQL 8.0.32)

• go test ./copydb/test ./sharding/test and go test ./test/go -run 'TestConfig|TestLogger': pass.
• test/integration/callbacks_test.rb: 3 tests, 0 failures.
• The smoke table above, run against the local containers with main and this branch.