fix(amazonq): limit maximum size of supplementalContext being sent (#5877)

## Problem
We found there were lots of cases where open tabs context sent from the
plugins with total length > 10240 chars long, which will result in
`validationException` error while users trigger inline completion
service.

## Solution
Limit maximum length of supplemental context added in the result set and
meanwhile Q team is conducting investigation to root cause it.

---

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License: I confirm that my contribution is made under the terms of the
Apache 2.0 license.

Author: Will-ShaoHua

packages/amazonq/.changes/next-release/Bug Fix-279c3a22-246e-41ca-9a42-3e446cdb9565.json

@@ -0,0 +1,4 @@
+{
+	"type": "Bug Fix",
+	"description": "Inline Suggestions: Occasional `ValidationException` if user context has too many characters."
+}

packages/core/src/codewhisperer/models/constants.ts

@@ -712,6 +712,7 @@ export const crossFileContextConfig = {
     numberOfChunkToFetch: 60,
     topK: 3,
     numberOfLinesEachChunk: 50,
+    maximumTotalLength: 20480,
 }
 
 export const utgConfig = {

packages/core/src/codewhisperer/util/supplementalContext/crossFileContextUtil.ts

@@ -132,9 +132,16 @@ export async function fetchSupplementalContextForSrcV1(
 
     // Step 4: Transform best chunks to supplemental contexts
     const supplementalContexts: CodeWhispererSupplementalContextItem[] = []
+    let totalLength = 0
     for (const chunk of bestChunks) {
         throwIfCancelled(cancellationToken)
 
+        totalLength += chunk.nextContent.length
+
+        if (totalLength > crossFileContextConfig.maximumTotalLength) {
+            break
+        }
+
         supplementalContexts.push({
             filePath: chunk.fileName,
             content: chunk.nextContent,