ICU-23513 ICU4J: fix PDI overrides and BD16 bracket-stack overflow

[roozbehp]

J1: Apply the active directional override to PDI as required by X6a,
while retaining its isolate identity for X10 and whitespace treatment for L1.

J2: Enforce BD16's limit of 63 pending opening brackets. On overflow,
discard all pairs in that isolating run sequence, including earlier pairs.
Track BD16 separately from ICU's N0 state; reprocess only when overflow
requires undoing earlier bracket resolutions.

Add regression tests for both issues, including matched/unmatched PDIs,
L1 line endings, the 63/64 boundary, canonical bracket equivalents,
isolating run sequence boundaries, contextual paragraphs and streaming.

Checklist

• Required: Issue filed: ICU-23513
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable
• Approver: Feel free to merge on my behalf

[roozbehp]

@markusicu, the fixes are not trivial, and would look like this. I can go ahead and create another pull request for the C bidi implementation, but you definitely need an owner on the ICU side to review and own this. It can't be me.

[roozbehp]

Converting to draft in favor of smaller more-reviewable changes.

[roozbehp]

Fix for the easier case at #4205