fix: parse union type annotations on tag variables

[DylanPiercey]

Problem

A union type annotation on a tag variable fails to parse:

<let/value: string | null = null/>

The | prematurely terminates the tag variable at value: string, and the trailing | null = null cascades into EOF reached while parsing expression. This blocks the two most common TS unions (T | null, T | undefined). The equivalent already works for generics (Array<string>) and intersections (A & B), and a parenthesized union (string | null) also works — only the bare union | was broken.

Fix

shouldTerminateConciseTagVar / shouldTerminateHtmlTagVar treated CODE.PIPE as an unconditional terminator, while CODE.OPEN_ANGLE_BRACKET returns !expression.inType (so < only terminates outside a type) and & is absent entirely. Make | mirror <: return !expression.inType. This only changes behavior inside a type annotation — outside a type, !inType is still true, so | terminates exactly as before.

Tests

New fixture tag-var-type-union covering bare unions in both concise and HTML tag-var forms (string | null, A | B | C, number | undefined) — each captures the full union as tagVar.value with the initializer kept separate. Full suite passes with no existing snapshot changes; tsc clean.

This is the root-cause fix for the union-type-annotation issue tracked in the marko repo (where it surfaces as the EOF reached while parsing expression cascade on <let/d: string | null = null>).

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.95%. Comparing base (93850d2) to head (3538b79).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #230   +/-   ##
=======================================
  Coverage   99.95%   99.95%           
=======================================
  Files          34       34           
  Lines        4270     4270           
  Branches      793      793           
=======================================
  Hits         4268     4268           
  Misses          2        2           

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[DylanPiercey]

Closed: this change is unsound. | is overloaded as both the TS union separator and the tag-body-parameters delimiter, so making | non-terminating inside a tag-variable type annotation breaks a valid pattern — a typed tag variable followed by body params, e.g. <my-list/ref: Api|item, index|> (parses as tagVar ref: Api + params item, index before the change; the |item, index| is swallowed into the type after). Requiring (string | null) is intentional disambiguation. Better path for the original issue: a diagnostic suggesting parentheses.