fix: reject full-width space and tab in Bedrock document file names

[takke1986]

Description of Changes

convertToSafeFilename sanitizes attachment file names for Bedrock's DocumentBlock.name, but the character class allowed \s:

nameWithoutExt.replace(/[^a-zA-Z0-9\s\-()[\]]/g, '_')

In JavaScript \s also matches U+3000 (ideographic space) and tabs, which Bedrock rejects. Kanji and kana were correctly replaced with _, so only whitespace characters slipped through the sanitizer. A file named テスト　資料.xlsx therefore failed with:

ValidationException: The document file name can only contain alphanumeric characters,
whitespace characters, hyphens, parentheses, and square brackets. ...

Full-width spaces are common in Japanese file names, so this is hit frequently. The reporter of #1671 measured about 19% of attached documents failing for this reason.

This PR:

• allows only the ASCII space instead of \s
• collapses consecutive spaces, which Bedrock also rejects
• falls back to a placeholder when the sanitized name would be empty (name has a minimum length of 1)
• applies the same fix to the AgentCore path in packages/web/src/utils/strandsUtils.ts, which had the identical \s in its character class

Not in scope: the file name length limit mentioned in the issue. The exact limit is not documented, so I left it out rather than guess. Happy to add it as a follow-up if you would like.

Compatibility: names that were already accepted are unchanged. Only names containing non-ASCII whitespace or consecutive spaces produce a different result, and those previously failed the API call outright.

Verified against the Bedrock API

I called bedrock-runtime:Converse in ap-northeast-1 with jp.anthropic.claude-haiku-4-5-20251001-v1:0, varying only document.name, to confirm that the names this function used to emit are actually rejected and the new ones are accepted.

document.name	Produced by	Result
test　name (U+3000)	before	ValidationException
test_name_706fc6f2	after	accepted
test name (two ASCII spaces)	before	ValidationException
test name_695fce01	after	accepted
test\tname (tab)	before	ValidationException
_______9c852928 (all-Japanese name)	after	accepted
file_d41d8cd9 (empty-name fallback)	after	accepted
test name (single ASCII space)	control	accepted

This independently reproduces the table in #1671.

Checklist

• Modified relevant documentation — n/a (internal utility; the JSDoc on the function was updated to state the actual constraint)
• Verified operation in local environment
• Executed npm run cdk:test — 45 tests / 15 snapshots passed, no snapshot differences

Also run:

• npm -w packages/cdk test -- fileNameUtils — 15 passed (9 existing + 6 added)
• npm run web:test — 278 passed
• eslint and prettier --check clean on all changed files

Related Issues

• convertToSafeFilename が全角スペースを素通りさせ、日本語ファイル名の添付が失敗する #1671
• 添付が失敗しても原因が利用者に伝わらない（ValidationException が全て UNKNOWN_ERROR に落ちる） #1672 is the reporting side of the same failure (the cause is not surfaced to the user); not addressed here.