processor/switch: add continue option for conditional case chaining

[twmb]

Summary

• Adds a continue field to switch processor cases
• Unlike fallthrough (which unconditionally executes the next case), continue passes the message back to be tested against the next case's check condition
• This mirrors the existing continue option on the switch output
• Setting both continue and fallthrough on the same case is a validation error

Test plan

• Test: basic continue behavior (message re-evaluated by next case's check)
• Test: continue message that doesn't match next case falls through
• Test: continue + fallthrough mutual exclusion validation
• All existing switch processor tests pass

Closes redpanda-data/connect#1089

🤖 Generated with Claude Code

[claude]

Commits
LGTM

Review
Adds a continue field to switch processor cases that re-evaluates continued messages against subsequent case conditions, complementing the existing fallthrough behavior. Mutual exclusion validation, core logic, and test coverage all look solid.

LGTM