fix: Resolve : ambiguity in URL path placeholders

[simon446]

Summary

:name path placeholders were ambiguous when followed by a literal : in the same path segment. The URL grammar matched the placeholder name greedily until /?#, and the Rust resolver's anchor required /?#$ after the name — so /users/:id:literal parsed as one big placeholder and never substituted. This PR tightens the grammar so the name excludes :, extends the resolver to accept : as a trailing boundary, and adjusts the editor (chip widget filter, highlight, param-list extraction) so the new boundaries are reflected visually.

Before / after

URL field rendering:

Before	After

Submission

• This PR is a bug fix or small-scope improvement.
• I have read and followed CONTRIBUTING.md.
• I tested this change locally.
• I added or updated tests when reasonable.

Approved feedback item (required if not a bug fix or small-scope improvement):

Related

:-suffixed path segments show up in the wild for AIP-136 custom methods (e.g. /tasks/{id}:cancel, /users/{id}:archive) and any other path style that uses :verb after a resource id. Before this PR, those URLs rendered with one greedy chip and didn't substitute when sent.

Details

Grammar (url.grammar): Placeholder { ":" ![/?#:]+ } excludes : from the name, and a /-segment now allows Placeholder PathSegment? so /:abc:def parses as Placeholder(:abc) + PathSegment(:def).

Resolver (crates/yaak-http/src/path_placeholders.rs): added : to the trailing-boundary regex set so :id in :id:literal substitutes correctly. One new unit test.

Editor:

• pathParameters.ts: chip widgets only render for Placeholder nodes whose preceding document character is / — filters out the lexer's mid-segment over-emission and the long-standing env-var-followed-by-:abc quirk (${[var]}:abc no longer renders :abc as a spurious chip).
• highlight.ts: dropped the Placeholder: t.emphasis style — chip widgets fully replace the placeholder text, so the underline was only ever visible on the spurious nodes we now skip.
• pathPlaceholders.ts (new): shared extractPathPlaceholders helper with unit tests, used by both HttpRequestPane and WebsocketRequestPane to keep the URL → params sync consistent.
• url.test.ts (new): grammar-level test documenting the lexer over-emission the chip filter relies on.

Test plan

• cargo test -p yaak-http path_placeholder — 9/9 pass, includes new placeholder_followed_by_literal_colon
• vp test --run against apps/yaak-client/lib/pathPlaceholders.test.ts and apps/yaak-client/components/core/Editor/url/url.test.ts — 8/8 pass
• tsc --noEmit in apps/yaak-client — clean
• npm test — 148 passing across JS workspaces, no failures
• Manual smoke: built target/release/yaak-app-client and verified the editor visually for /:abc:def, ${[env]}:abc, and OpenAPI-imported requests (see Before / after above)