test: cover full per-offering config migration on issuer transfer

[ogbemercyada-sketch]

Add comprehensive config-migration test matrix for accept_issuer_transfer verifying that InvestmentConstraints, ClaimDelaySecs, SnapshotConfig, and LastSnapshotRef follow the offering under the new issuer identity.

Changes:

• Fix test_issuer_transfer_migrates_all_configs: correct set_concentration_limit signature (add missing max_staleness_secs=&0u64 arg), add assert for rounding mode defaulting to Truncation under old issuer, add descriptive failure messages
• Add config_migration_setup() helper to de-duplicate test boilerplate
• Add 10 new tests covering:
  • Partial config (only 2/7 set): unset configs remain at defaults after transfer
  • Transfer-then-set: new issuer can overwrite all configs post-transfer
  • Old-issuer exclusion: old issuer cannot set investment constraints, claim delay, or snapshot config after transfer (three separate boundary tests)
  • Fresh-registration isolation: old issuer registering a new offering after transfer starts with clean defaults, not contaminated by migrated-away keys
  • Large snapshot ref boundary: u64 value 0xFFFF_FFFF_FFFF survives migration
  • Both-zero investment constraints (min=0, max=0) migrate correctly
  • Zero claim delay set explicitly still yields 0 under both issuers
  • Multi-namespace isolation: ns_a transfer does not affect ns_b configs
• Apply cargo fmt --all to fix pre-existing formatting drift in lib.rs, tax_bucket.rs, and test_tax_year.rs so CI fmt check passes

Closes #844

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