Fixing exception in String .isSplittable

[VardhanThigle]

Overview

There's a bug in String handling that causes a typecasting exception which checkin if a string is splittable.

Root Cause of the Bug

For string, the boundary extractor was created of type long. This did not trigger while testing a small migration of few strings in different languages as there was no need to check if the range is splittable.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 49.46%. Comparing base (bbf744f) to head (538071b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1755 +/- ## ============================================ + Coverage 42.58% 49.46% +6.87% + Complexity 3364 1092 -2272 ============================================ Files 815 337 -478 Lines 47672 18204 -29468 Branches 5107 1844 -3263 ============================================ - Hits 20301 9004 -11297 + Misses 25717 8593 -17124 + Partials 1654 607 -1047 ``` | [Components](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1755/components?src=pr&el=components&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | Coverage Δ | | |---|---|---| | [spanner-templates](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1755/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `64.64% <100.00%> (+0.07%)` | :arrow_up: | | [spanner-import-export](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1755/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `∅ <ø> (∅)` | | | [spanner-live-forward-migration](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1755/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `75.00% <ø> (ø)` | | | [spanner-live-reverse-replication](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1755/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `51.96% <ø> (ø)` | | | [spanner-bulk-migration](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1755/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `83.45% <100.00%> (ø)` | | | [Files](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1755?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | Coverage Δ | | |---|---|---| | [...niformsplitter/range/BoundaryExtractorFactory.java](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1755?src=pr&el=tree&filepath=v2%2Fsourcedb-to-spanner%2Fsrc%2Fmain%2Fjava%2Fcom%2Fgoogle%2Fcloud%2Fteleport%2Fv2%2Fsource%2Freader%2Fio%2Fjdbc%2Funiformsplitter%2Frange%2FBoundaryExtractorFactory.java&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform#diff-djIvc291cmNlZGItdG8tc3Bhbm5lci9zcmMvbWFpbi9qYXZhL2NvbS9nb29nbGUvY2xvdWQvdGVsZXBvcnQvdjIvc291cmNlL3JlYWRlci9pby9qZGJjL3VuaWZvcm1zcGxpdHRlci9yYW5nZS9Cb3VuZGFyeUV4dHJhY3RvckZhY3RvcnkuamF2YQ==) | `100.00% <100.00%> (ø)` | | ... and [494 files with indirect coverage changes](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1755/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform)