fix(@angular/ssr): resolve parameters in redirect targets

[bschaeublin]

PR Checklist

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• The commit message follows our guidelines: http://github.com/angular/angular-cli/blob/main/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Follow-up to #34301 (comment).

Parameters in a redirectTo target are sent unchanged in the server's Location header, while the router replaces them:

Route	Request	Server Location	Router navigates to
user/:id → profile/:id	/user/7	/profile/:id	/profile/7
search/:term → /find?q=:x	/search/cats?x=dogs	/find?q=:x	/find?q=dogs

What is the new behavior?

The server resolves both, one commit each:

1. Route parameters: during route extraction, :name in the redirect path becomes *<n>, the position of that parameter in the request path, which buildPathWithParams resolves at request time. Prerendered routes get the value from getPrerenderParams.
2. Query placeholders of absolute redirects: q=:x takes the values of x from the request query.

Does this PR introduce a breaking change?

• Yes
• No

Other information

Without x in the request, the server redirects to /find. The router would go to /find?q=undefined.

[gemini-code-assist]

Code Review

This pull request enhances Angular SSR's redirect handling by resolving route and query parameter placeholders in redirect targets, aligning its behavior with the Angular router. It introduces buildQueryWithParams to resolve query placeholders, supports positional placeholders (*) in buildPathWithParams, and adds corresponding unit tests. The review feedback highlights a potential runtime error in buildPathWithParams when positional placeholder indices are out of bounds, suggesting a fallback to empty strings. Additionally, it recommends using .startsWith(':') instead of direct index access [0] in both replaceRedirectParams and buildQueryWithParams to safely handle empty strings.

[gemini-code-assist]

If fromPath has fewer segments than toPath, or if the positional placeholder index is out of bounds, fromPathParts[index] or fromPathParts[position] will be undefined. This will cause joinUrlParts to throw a TypeError when accessing part.length on the undefined value, or result in the string 'undefined' being inserted into the path. Adding a fallback to an empty string prevents these runtime errors.

Suggested change

	const resolvedParts = toPathParts.map((part, index) =>
	toPathParts[index] === '*' ? fromPathParts[index] : part,
	part === '*'
	? fromPathParts[index]
	: part.replace(POSITIONAL_PLACEHOLDER_REGEXP, (_, position) => fromPathParts[position]),
	);
	const resolvedParts = toPathParts.map((part, index) =>
	part === '*'
	? (fromPathParts[index] ?? '')
	: part.replace(POSITIONAL_PLACEHOLDER_REGEXP, (_, position) => fromPathParts[position] ?? ''),
	);

[bschaeublin]

Not reachable: each */*<n> placeholder is derived from the position of a parameter in the matched route, and the route tree only matches when the request has a segment at each of those positions.