Import-DbaCsv - Fix RFC 4180 multiline quoted field handling

[mbentham]

Summary

Fixes two bugs that caused Import-DbaCsv to reject valid CSV files containing multiline quoted fields (permitted by RFC 4180).

• Pre-validation false rejection (Import-DbaCsv Pre-validation incorrectly rejects valid CSV files with multiline quoted fields #10188): The delimiter check used -notmatch on lines read by Get-Content, which splits on newlines without understanding CSV quoting. If any sampled line lacked the delimiter (e.g. a continuation line from a multiline quoted field), the file was rejected. Changed to -match so the file is only rejected when no sampled lines contain the delimiter.

• Multiline fields disabled by default in Strict mode (Import-DbaCsv Multiline quoted fields cause parse errors by default with RFC 4180 Strict mode #10189): AllowMultilineFields defaulted to $false despite QuoteMode defaulting to Strict ("RFC 4180 compliant"). RFC 4180 explicitly allows CR/LF inside quoted fields. Now AllowMultilineFields defaults to $true when QuoteMode is Strict, unless -SupportsMultiline is explicitly provided. Applied consistently across all three CsvReaderOptions construction sites (table creation, schema inference, and main import).

Fixes #10188
Fixes #10189

Test plan

• Import a CSV where the first data row starts with a multiline quoted field (triggers Bug 1)
• Import a CSV with multiline quoted fields deeper in the file without -SupportsMultiline (triggers Bug 2)
• Import a standard CSV without multiline fields (no regression)
• Import with -QuoteMode Lenient confirms multiline stays off unless -SupportsMultiline is passed
• Existing Import-DbaCsv tests pass

(do Import-DbaCsv)

[potatoqualitee]

Looks reasonable, thank you very much for the fix!