fix: respect user-specified User-Agent headers without modification

[robert-j-y]

Description

Fixes #300 - When users provided a custom User-Agent header, the SDK was appending its identifier to it, resulting in unexpected behavior like my-app/1.0, ai-sdk/openrouter/x.x.x.

Before:

createOpenRouter({ headers: { 'User-Agent': 'my-app/1.0' } })
// Sent: "my-app/1.0, ai-sdk/openrouter/x.x.x"

After:

createOpenRouter({ headers: { 'User-Agent': 'my-app/1.0' } })
// Sent: "my-app/1.0"

The fix also addresses a case-sensitivity bug where User-Agent (capitalized) was not recognized as the same header as user-agent (lowercase), causing duplicate headers to be sent.

Key changes:

• Added case-insensitive header lookup for User-Agent via findHeaderKey()
• User-specified User-Agent headers are now used verbatim
• SDK identifier is only added as default when no User-Agent is provided
• Added normalizeHeaders() to properly handle all HeadersInit variants (Headers objects, array-of-tuples, plain objects)
• Fixed removeUndefinedEntries to filter both null and undefined values (was only filtering null)
• Added comprehensive unit tests (14 tests) for withUserAgentSuffix

Updates since last revision:

• Added normalizeHeaders() function to handle Headers objects and array-of-tuples inputs (previously Object.entries() on a Headers object returned empty array, silently losing all headers)
• Fixed removeUndefinedEntries to use value != null instead of value !== null to properly filter undefined values
• Added tests for HeadersInit variants (Headers object, array-of-tuples)
• Updated undefined test to verify key is actually removed from result object

For reviewers:

• Verify the behavior change is acceptable (SDK identifier no longer appended when user provides User-Agent)
• Review normalizeHeaders() handling of Headers objects and arrays
• Note: removeUndefinedEntries is only used by withUserAgentSuffix, so the fix is isolated

Checklist

• I have run pnpm stylecheck and pnpm typecheck
• I have run pnpm test and all tests pass
• I have added tests for my changes (if applicable)
• I have updated documentation (if applicable)

Changeset

• I have run pnpm changeset to create a changeset file

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Link to Devin run: https://app.devin.ai/sessions/27723ec908ef4455be28e3c3c491b31e
Requested by: Robert Yeakel (@robert-j-y)