[Server] Fail when discovery dependency is missing

[ousamabenyounes]

Summary

• fail immediately when file-based discovery is configured without symfony/finder
• preserve custom discoverers, which do not require Finder
• cover the missing optional dependency in an isolated regression test

Fixes #398.

Test verification (RED → GREEN)

Reproduced on unmodified main with production-only dependencies:

BUG: build succeeded with Mcp\Server

With the regression test present and the production fix reverted (RED):

Failed asserting that exception of type "Mcp\Exception\RuntimeException" is thrown.
FAILURES!
Tests: 1, Assertions: 2, Failures: 1.

With the fix applied (GREEN):

OK (1 test, 3 assertions)

Full local validation:

Composer validation: passed
PHP CS Fixer: passed
PHPStan: passed
PHPUnit unit + integration: OK (1237 tests, 3384 assertions)
phpDocumentor: passed
Changed executable line coverage: 2/2