🐛 Fix ListFormat.tsv joining values with a literal \t - #2628

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Additional context and info (if any)

Closes #2627

ListFormat.tsv is documented as tab-separated, but _getSeparatorChar returned the raw string r'\t' (a backslash and a t), so values were joined with those two characters instead of a tab. In query strings this put a raw backslash into the URL (a=foo\tbar); in urlEncodeMap it became a=foo%5Ctbar.

The separator is now a real tab: %09 in query strings (matching how ssv already uses %20 there) and \t otherwise, which urlEncodeMap encodes to %09 and FormData keeps as a raw tab.

@AlexV525 calling this out because it changes request encoding on the wire for ListFormat.tsv users. Anyone currently relying on the literal \t would see real tabs instead, which is what the format is documented to produce.

The three existing tsv expectations in encoding_test.dart (urlEncodeMap tsv and custom, urlEncodeQueryMap custom) had captured the wrong output; they now expect %09 and fail without this change. dart test -p vm in dio/ passes (253 tests; the TLS-tagged tests are skipped by the default preset), and dart format and dart analyze --fatal-infos are clean.

Implementation and tests by Claude (Anthropic); reviewed by me before submitting.