Add typed Filter builder for collection filtering

[loevgaard]

Summary

Replaces hand-rolled filter strings like

$options->withFilter(sprintf(
    'lastUpdated$gte:%s',
    $since->setTimezone(new \DateTimeZone('UTC'))->format('Y-m-d\TH:i:s\Z'),
));

with a typed builder:

$options->withFilter(Filter::gte('lastUpdated', $since));

• Setono\Economic\Request\Filter (final readonly, \Stringable, Identifier-style private constructor): static factories eq/ne/gt/gte/lt/lte (string|int|float|bool|\DateTimeInterface|null), like (* wildcard preserved), in/nin (non-empty list, max 200 elements per e-conomic's cap, null elements render as $null:), and raw() as the verbatim escape hatch for anything the factories can't express.
• Special characters in values ($ ( ) * , [ ]) are $-escaped automatically via single-pass strtr; null renders as $null:; any DateTimeInterface is converted to UTC and formatted Y-m-d\TH:i:s\Z without mutating the input; non-finite floats are rejected.
• ->and()/->or() parenthesize every composite operand (receiver included) since e-conomic doesn't document $and:/$or: precedence; raw() filters are treated as composite when combined.
• CollectionRequestOptions narrows to ?Filter (constructor and withFilter()) — raw strings are no longer accepted. Pre-1.0 break (latest tag is v1.0.0-alpha); hand-written expressions go through Filter::raw().
• README, openspec/specs/endpoint-api/spec.md, and CLAUDE.md updated to the new API.

Test plan

• composer phpunit — 232 tests, 785 assertions, including a new end-to-end test proving the unencoded-expression + RFC 3986 query-encoding layering through ScriptedHttpClient
• composer analyse — PHPStan level max, clean
• composer check-style — clean
• vendor/bin/rector --dry-run — clean
• vendor/bin/infection — MSI 17 points above the configured minimum