feat(@angular/cli): allow Algolia search key override via env var

[gn00295120]

PR Checklist

• The commit message follows our guidelines: https://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) — no docs are touched; the only existing env-var doc is NG_DEBUG in docs/design/analytics.md, which is for a different purpose. Happy to add a brief mention if reviewers prefer.

PR Type

• Feature

What is the current behavior?

The MCP search_documentation tool uses a single bundled Algolia API
key for the public Angular documentation index. Operators have no way
to substitute a different key for self-hosted documentation, internal
CI environments, or rotation testing without rebuilding the CLI.

Issue Number: N/A

What is the new behavior?

A new environment variable, NG_DOCS_SEARCH_API_KEY, takes precedence
over the bundled key when set to a non-empty value. When unset or set
to an empty string, behaviour is unchanged.

The override path is factored into a small resolveAlgoliaApiKey()
helper so the precedence rules are unit-testable without spinning up
the full MCP tool runner. The helper is exported (for testing) and
consumed from createDocSearchHandler:

export function resolveAlgoliaApiKey(): string {
  const override = process.env['NG_DOCS_SEARCH_API_KEY'];
  if (typeof override === 'string' && override !== '') {
    return override;
  }
  // Fall back to the bundled default (existing decryption code path).
  ...
}

The bracket-syntax env access matches the existing convention in
mcp-server.ts (process.env['NG_MCP_CODE_EXAMPLES']). The variable
name follows the NG_* prefix used by every other CLI environment
variable (see utilities/environment-options.ts).

Tests

Three Jasmine specs in
packages/angular/cli/src/commands/mcp/tools/doc-search_spec.ts:

1. returns the env var value when set to a non-empty string
2. falls back to the bundled default when the env var is unset
3. falls back to the bundled default when the env var is an empty string

The fallback specs assert the resulting key matches the expected
Algolia key format (32 hex chars) without logging the value.

Comment cleanup

The previous comment block on ALGOLIA_API_E is replaced with a
short note describing the new override mechanism and the operator
scenarios it enables. The /#search-only-api-key URL was tied to
that specific phrasing; since the new comment no longer makes that
claim, the URL is dropped to avoid implying a constraint the bundled
key does not necessarily honour.

Does this PR introduce a breaking change?

• Yes
• No

Default behaviour is unchanged for users who do not set the new
environment variable. Existing ng mcp workflows continue to use the
bundled key with no configuration.

Other information

The variable name choice (NG_DOCS_SEARCH_API_KEY vs alternatives
like NG_ALGOLIA_API_KEY or NG_MCP_DOCS_KEY) is a judgement call;
happy to rename to whatever fits the convention reviewers prefer.

[gemini-code-assist]

Code Review

This pull request introduces the resolveAlgoliaApiKey function to allow overriding the Algolia API key via the NG_DOCS_SEARCH_API_KEY environment variable, falling back to a bundled default if unset. It also includes unit tests for this new logic. Feedback suggests providing an environment variable override for the Algolia App ID to better support self-hosted documentation and recommends trimming whitespace from the environment variable value to prevent configuration errors.